Sure, I'd love to talk about it! I don't think there's ever been official information about it, but having played the game since early HW and paid attention to things like job popularity, I've noticed a few trends in how job popularity tends to go. The statistics I mention here are a combination of anecdotal experience and historical log data from repositories like Lucky Bancho Census and FFLogs.
It's always a little hard to say because different audiences look for different things for jobs -- for instance, Samurai's current popularity seems to be a synthesis of three things: the appeal of the katana-wielding fantasy archetype, its mechanical strength in raids, and the appeal of having some of the highest potency single target nukes in the game. But it wasn't always so well loved! In Stormblood, when SAM had come out, it was insanely popular. But by the end of the expansion, it was considered a niche, weak job that not many people ran with, because it was a weird job with strange 35 and 40s timers, and using Midare if Hagakure was up was bad actually, and it didn't have raid buffs or fit that great into them, et cetera, et cetera. It still had the massive single target nukes and the katana-wielding fantasy archetype, but that wasn't enough to carry it above the likes of Dragoon and Bard (which were some of the most popular jobs that expansion).
Compare that to late EW, after SAM had its complexity shaved down to fit into the two minute meta and be more approachable to the casual audience, and suddenly SAM has become the most popular melee among raiders and nearly the most popular among the casual audience as well. What happened? Well, simplification happened, and players seem to have responded well to it. SAM is way more popular than it ever was before, barring the 4.0 launch patch. Now, this is also partially because the addition of Dancer made Samurai way better, but still, when the only jobs competing with Samurai for popularity are jobs like Reaper, which was weak throughout nearly all of EW and still insanely popular, raid strength seems to take a back seat to approachability and appeal.
You can see this effect in something like SMN, too, which was a weird, wild, wacky DPS with clunky pet mechanics, a magic vuln up tied to Garuda Egi's weird cast times, and a Bahamut phase that involved weaving Addle because it made Bahamut attack. Both BLM and SMN were considered difficult jobs to play, and RDM completely siphoned all of their popularity from the casual audience at pretty much all times. And now, post-EW's absolute genocide of SMN's complexity, it's the most popular caster. RDM is considered awkward in comparison with tricky mobility despite not having changed really at all since its introduction, and part of this is because RDM has held onto a lot of its complexity, between things like avoiding Fleche drift, proc management, double- and triple-combo burst windows, etc. When RDM is such an insanely popular class fantasy (who doesn't love spellswords), how does it lose out to SMN in terms of playerbase representation? Well, it certainly helps that anyone and their mom can play SMN practically without friction.
I want to talk about Reaper a little, too, because while it has a good deal less history than these other jobs, it nonetheless provides insight into how Square tends to try and make popular jobs approachable, and vice versa. Now, Reaper has never been "hard" (non-standard stuff aside); if you could do Double Enshroud under burst (which involves getting two Communio casts into raid buffs), you could pretty much play the job in any scenario. This, combined with its incredibly appealing class aesthetic (scythes are cool and popular) made it the most popular melee in EW. What's interesting about this is, during Dawntrail, the devs seem to have tried to kill Double Enshroud. It's a complex subject, but the buffs to Gluttony and the addition of Perfectio seem to have been designed to make a standard Enshroud phase too long to fit 2 under raid buffs. Now, they failed, and 2 Enshroud is still quite doable, but the intention makes it clear that, seeing Reaper's popularity, they wanted to make it even more approachable to cement that popularity. I mean, in the Job Action Live Letter, they described their Reaper changes as designed to "improve ease of use" to one of the easiest jobs, which says a lot about their intent.
As for who the developers design jobs to cater to, we can't tell what comes first between the chicken and the egg: Do they make jobs they think will have popular class fantasies simple on purpose? Or do they simplify the most popular classes so even more people will be able to play them? In my opinion, the answer is probably "both," but it puts us in a weird position.
Job fantasies like "the avatar of death," "far eastern/Vergil samurai guy," "monstrous battle rager too angy to die," etc. are obviously more appealing than more niche things like "magic casting Ninja" and "half bowman half bard," so we see a lot of popularity in those jobs. And, perhaps because they are less popular, jobs like Ninja and Bard are able to retain their complexity--because not enough people care enough to pick up and main Ninja in the first place, let alone start complaining about how it plays, it has retained a position as a complex, burst optimization job pretty much since its conception.
But Viper is weird, because its job identity, to me, is one that potentially falls into this "more niche appeal" category. Tural will not be the setting of the next 10 years of FFXIV, and while dual swordsman is a very popular weapon fantasy, XIV-original hunter/ranger/rogue hybrid with explicit links to a particular setting may not be as much in Final Fantasy, where legacy jobs tend to have a huge draw as well. In 2 years, when Viper is not a brand new job, the flagship in the trailer, and the only job associated with the current setting, will its class fantasy be enough to endear it to people? I'm not sure.
If its class fantasy is extremely appealing to a wider audience, then player data suggests that the objectively correct call from SQEX is to eliminate as much friction within the job as possible and make it for babies. That's what worked for everything else on this list. But if Viper's appeal is more like Ninja's, where it will appeal to a core (and largely gameplay driven) audience, the right choice would be to maintain depth and complexity and give those players something to dedicate to. We won't know which one was right until a couple years out, because any argument about Viper's popularity right now is stained by the fact that it's the brand new flagship job and literally everyone is playing it, but we can speculate.
In my opinion, Viper was initially designed to be targeted to midcore, dedicated audiences. The fandom response when it was revealed was very much so "That's it?/I wanted Corsair," and it was described as a fast-paced, technical-but-stylish job. Many predicted Picto would be more popular. But after seeing the absolute inundation of Vipers in the PF (for EX1 and EX2 clears, there are literally twice as many Vipers as there are Dragoons, Samurais, and Monks COMBINED), Square realized that the job has massive casual appeal and that's where these changes are coming from They want to eliminate friction for a sudden and unexpected cash cow. I think this is a risky move on their part, because in 2 years who knows how many people will be itching to play Viper, but the intent seems clear. I hope they dial back and wait longer to see what the "right call" is (and as an Ultimate Raider and hardcore player, I hope they choose to keep it more complex), but existing precedent scares me a little.
Thanks for reading!