The main reason the turnwheel exists is because Fire Emblem does not really abide by its old design standards anymore. It hasn't for a while. Echoes is ironically a really good step in the right direction for ironman runs (alternate dialogue for characters dying, mourning quotes, etc) but also at the same time it introduces the turnwheel, which seamlessly fixes the long-glaring problem that the series has had for a very long time now: Players deliberately going against the game's design philosophy.
FE was originally designed to be beatable under ironman conditions; it was the entire expectation that you'd lose units and have to replace them. That's why most FE games throw tons of units at you, several very similar to units you got earlier on. It's why the difficulty has always been rather on the low side, and games with smaller casts usually had mechanics to accommodate this idea; Sacred Stones has grinding and infinite exp with a lower difficulty bar than usual because it has the smallest cast in the entire series. Losing a character in SS technically hurts substantially more than in any other game, so they provided safety net mechanics in case the player actually needed to go back and train up a benched unit because somebody important died.
Even then, most FE games have really good prepromotes that can carry you through the game, too. Most people just choose not to use them as a "house rule" and things like not using the turnwheel aren't any different, but that's getting into a different topic.
The reason the turnwheel exists is the fault of the players, who have for ages now restarted the chapter when things didn't go their way instead of pushing on and accepting their losses. The most common line of thought is that they're not "winning" unless they come up with a flawless strategy. But that's not what FE is about, it's not what FE has ever been about till recently. FE was always designed under the philosophy of "Don't design expecting perfection, design to accommodate failure." But newer games, mostly just Awakening and especially Fates, have thrown this philosophy out the window. They don't expect you to lose any units ever and the fact classic mode even exists is basically just a holdover from the rest of the series, a mechanic grandfathered in that is hardly respected anymore. Losing a unit in Fates for example, or at least particularly in Conquest, just punishes you needlessly hard and in some cases can make things actually unwinnable without needlessly elaborate strategies or abusing broken mechanics. Losing a unit in Fates or Awakening can potentially rob you of another one entirely (as anyone who's ever lost Kaze out of nowhere to scripted bullshit would know) and it also deprives you of all their supports in a game where supports are stressed even more than normal, and it's just a downward spiral where you only carry on if you're pushing yourself, not because it's how the game expects you to play.
The turnwheel is the embodiment of this new-age reset playstyle streamlined as a core mechanic of the game, but without undermining the principal of loss either. You could rewind and fix your mistake, but you don't have to. You can carry on anyway if you want to, which is the usual defense people give of Classic Mode's existence to begin with: they usually restart, but like the idea of not having to restart as if everyone was a forced gameover. The turnwheel is no different. It has no reason to go away and has all the reason to stay, the only thing I'd hope would be that it would have substantially reduced charges since in Echoes, the charges mainly existed to ease long dungeons where the charges didn't reset. You shouldn't be able to brute force your way using the turnwheel, it should be a resource you have to manage and respect just like you would for things like the durability of your legendary weapons, for example.
Does the turnwheel partly exist to alleviate #GaidenMapDesign? Yes, it most likely does. But that isn't a bad thing, either, really, since it's a mechanic that honestly should have existed sooner. It's far better than the battle saves system that RD had, where you'd just slog through a chapter and break your momentum entirely if you messed up. The turnwheel is seamless and integrates the experience so much more. It doesn't feel like you're exploiting a mechanic in the game so much as you're using the tools given to you to pull off the best strategy you can, and it doesn't impact the flow of the chapter because it's so cleanly integrated, with how it just slides in and lets you flick back and forth through actions without so much as interrupting the BGM. A lot of people don't seem to understand how much this does for gameplay, especially during the final battle where you've got this long, beautiful vocal song that overlaps all phases to carry atmosphere.
It's an elegant combination of form and function that does wonders for the gameplay and it is the realistic progression of the series given that its playerbase shows so little interest in abiding by the old standards, but it doesn't invalidate those who wish to still play by them, because at the end of the day it's a completely optional and noncommittal mechanic, highly contrary to the very often heavily panned casual mode that takes most of the value of strategy out of the game. It's a seamless bridge between classic and casual; you can use it to help yourself out of bad moves, or you can restrict yourself and only use it if you feel like the game has cheated you, or you could ignore it entirely, or use it under arbitrary conditions of your own. It's not a system that locks you in, like casual mode does, since it's a game mode option presented to you before you even start the game, and it doesn't invoke the feeling of easy mode shaming either, something a lot of games have been trying to shake off lately. Even if you rely on it, it still makes you actually play the game and think, it just gives you an escape route if you screw up. Ideally, the next FE game won't even need casual mode, because it'll have a balanced turnwheel to fulfill that need anyway.
tl;dr: The Turnwheel is the natural progression of the series that streamlines the "reset when someone dies" methodology that has stuck with players for a long time, acts as a bridge between classic and casual, and just needs to be balanced a little bit so the player can't brute force their way through a chapter with it. There is no reason why it shouldn't become a staple of the series henceforth.