Again I don't agree.
At a quick glance, asking "why do non-raiders need similar ilvl gear to raiders" does look like a fair question. If you are not doing raids and world and dungeon mobs really don't get all that much tougher, why would you need to do 3k dps vs. 1k (making up numbers here)? The answer? First and Foremost this is a gear progression based game for the most part. Even casual players will get all their drops in dungeons or crafted gear eventually. And once that happens where do you go from there? Without a continuous goal to strive for, players end up drifting away. Case in point, the reduction of players waiting for Stormblood. Most of them likely have all the gear they need and have little to work towards in the lull before the xpac drops.
SE tries to get us content on a regular basis, but in all honesty they will never ever be able to produce it as quickly as we consume it. So it gives players something to work towards if raiding doesn't fit their play style. This is one of the reasons tomestone gear exists and is as good as it is. Those non-raiders have something to work towards after they've gotten what they can get from the content they engage in. Raiders also benefit from it as tomestone gear supplements their raid drops allowing them to make up for poor rng and/or get geared faster.
I've mentioned this before but player power is one of the biggest motivators to play the game and SE most certainly wants as many people playing the game as possible. Player power also doesn't exist in a vacuum. If the gear disparity is too great between raiders and non-raiders, then raiders have a large advantage in ALL content, not just raiding. Saying that the "majority" of raiders raid just to clear the content is BS. Because if that were true, we wouldn't be having this discussion right now, and raiders wouldn't have had a conniption fit over Gordias Savage and the Anima going to 275.
Ultimately the game as a whole benefits from keeping the gear gaps small. Why? Because, simply put, it eases the engaging of ALL content. As a raider, you'll invariably lose members of your static to a variety of reasons from burn out to babies. Getting new raiders to fill that spot isn't always easy, but a small gear gap does help in that regard in that your pool of potential recruits will typically not be too far behind gearwise for the raid content you're engaging in. In non-raid content, how many of us /facepalm when we see someone in our dungeon who's just gotten to a high enough ilvl to qualify for it? The runs go slower when the lower ilvl tank has only half the hp of the raid tank, or the low ilvl healer can't keep up with healing large pulls, or the low ilvl dps can't kill the packs fast enough for the tank and healer to last through. It's less fun for everyone. While that's inevitable to some degree, for the most part people don't stay at those ilvls for too long since the tomestone gear does allow them to catch up in a decent amount of time.
There's more to endgame in FFXIV than raiding and that's something that I think many people struggle with. FFXIV is not really a "raid or die" game. It's more a "raid if you want a small edge and access to exclusive cosmetics (armor models, mounts, etc)" type of game. The difference in player power due to gear is very small in FFXIV and thats a good thing! Why? Because it makes it so skill will make more of a difference in your performance than just having a huge gap in ilvl between players. It also gives players who aren't raiders something to strive for and facilitates their entry into raiding if they so desire.
If anyone can't see the benefits of the small gear gap we have in FFXIV right now, then I say they either aren't looking hard enough, or they're too self centered to see the bigger picture.