I don't think there's a problem with the philosophy of "play other stuff when you're done with FFXIV", at all. But it does exacerbate other issues with the game.
For clarity: The direct effect of the "play other games" philosophy is that A) there is no tangible benefit to gearing up in the current content cycle that isn't reset (by crafted gear, etc) before the next content cycle starts, B) there is little to no randomization in the gearing system, such that you can know almost exactly how many weeks it will take to be "done", and C) there is no tangible benefit to continuing to play within a content cycle once you already have BiS (no super-rare RNG-gated gear, relics are never relevant until x.5+, no special materia that costs weekly-capped tomestones to power you up past BiS, etc). Changing that philosophy would involve changing one of those three things, or enacting a similar change.
The problem, though, boils down to two things:
First, FFXIV is not designed, first and foremost, to be fun. Contrast this to games like Destiny 2, or Guild Wars 2, or even FFXI, and the difference is obvious: Even if you have nothing "to do" in those games for progression, they are designed to be fun. Running around the Cosmodrome shooting Dregs feels good, even if it's pointless and won't keep you busy for very long. Ditto for just running around getting into random, pointless fights in GW2, and ditto for doing any of the content in FFXI that doesn't expressly serve the purpose of gearing you up for later endgame. Within their respective genres and subgenres, those games are all designed to feel good and to exercise your ability to play and to improve. FFXIV is not designed this way. FFXIV is designed, first and foremost, to be functional.
The function of FFXIV's core gameplay, as far as the developers are concerned, is to make it very easy and very predictable for them to design raid encounters. The developers never need to take a hard look at the content and wonder what players will be able to achieve inside of it, whether they're asking too much or too little. There is almost zero room for the designers to make a mistake, so long as they colour inside the lines. Notice that a piddly 1% nerf to P8S was suddenly a big deal for them and for the community - it's because that's not supposed to be possible. Everything is supposed to be so locked-down and predictable that they never overshoot or undershoot by even 1%. They're supposed to only ever look forward and work on the next patch, or on some pre-approved overhaul like the Duty Support redesigns. They're never supposed to have to go back and make any changes to recent, existing patches. The function of FFXIV's core gameplay, for the developers, is to protect the sanctity of their production pipeline and schedule.
For FFXIV's players, the fun is meant to come entirely from the content. You're supposed to have FATE-style fun while doing FATES, you're supposed to have Dungeon-style fun while doing Dungeons, you're supposed to have Normal Mode Raid fun while doing normal modes, Savage Raid fun while doing Savage, Ultimate fun while doing Ultimate. There is not supposed to be crossover where you just enjoy playing your class in a way that is fundamentally identical across any level of content. Classes are designed to be functional in a way that facilitates having Dungeon fun in dungeons, Savage fun in Savage, etc.
This is intentional and has been an ongoing aspect of the game for years. SE have been working toward this since Stormblood, and they largely achieved those goals with Shadowbringers.
Second is an Endwalker-specific problem, which is that SE have just made extremely poor decisions about what content to include (and not include) in this expansion.
Despite being a pretty formulaic game, the exact content released in each expansion so far has been a little different. Shadowbringers didn't have a Deep Dungeon at all, Heavensward put more effort into its Deep Dungeon and less into its Exploration zone and Stormblood was the opposite, etc. But they've always done a decent job at striking a balance between different types of "non-core" content: There's always been super-challenging stuff (Savage SCoB, Gordias/Midas, Ultimates in subsequent expansions), there's always been longform grind content (usually the Relic), there's always been stuff aimed at people who want non-combat content (HW tried to make Specialists a thing, StB had the Doman Enclave, etc), and there's always been attempts to push the game a little in terms of injecting a little more fun and a little more sandbox into the game (HW centering it with complex class design, Eureka Logograms, Bozja Lost Actions).
The only thing they've arguably gotten right in Endwalker is the top-end stuff, putting out two Ultimates.
There's no longform relic grind for people who want that
There's no attempt to provide interesting sandboxed gameplay for people who liked Logograms/Lost Actions
The non-combat side of things is obscenely overbudgeted with Island Sanctuary; it seems like the Exploration zone budget was cannibalized to make a glorified 2008 Facebook game
It's not a production bandwidth issue like the crunch in ARR/HW or Covid hitting ShB, either; the actual level of production in Endwalker seems to be humming along at Stormblood speeds. The problem is that they've invested a ton of it into making unrepeatable, one-off side story content like whatever all that Tataru stuff is, and into Variant/Criterion dungeons, which have clearly had a lot of work put into them but have pretty much zero legs in terms of replayability and don't really distinguish themselves from the game's regular "core" content.
Add those together, and the outlook is really bleak for a lot of people who have otherwise always had good reasons to log on. Lots of people stayed attached to the game through having relic content to grind though; that's gone. I used to log on just to run dungeons in HW and Stormblood; that was gone in Shadowbringers but at least by the end I had Bozja to do where I could play around with Lost Actions and doing weird off-role things - now there's neither.
"Take a break from the game when you're done with it" is good. But the assumption is that you're not going to be "done with it" for at least a few weeks - and in Endwalker, I'm seeing (and experiencing) lots of cases where we're hitting that point after half a day, despite wanting to have a reason to keep playing.