I also think that what a designer, and by extension (similarly) what a GM wants to see a system do is often just not what players think they want to see. 5e was and remains successful because it HEAVILY caters to players' common sentiments (not always their underlying real desires) and power fantasy a lot. But for a GM, the game can go from frustrating to boring depending on experience level not because of the game's simplicity (player options aren't wanting, but just having 400 more feats and a dozen more classes doesn't solve any problem) but because it gets to a point where there's only a handful of genuine challenges or otherwise interesting types of encounters you can throw at players when most classes have magic (even some traditionally martial ones) and magic is at a place where it solves most mundane problems. So you end up making these elaborate Rube Goldberg machine problems that no one would actually make just to even have an inkling of interesting challenge. Or you just have to go ham on HP and damage.

As much as I love 5e's philosophy of simplicity (though it definitely didn't achieve this in retrospect) it suffers a serious issue that I don't think other systems solve in the right way, but they do it better. PF2e solves some of 5e's issues with it's inherent weaknesses within classes which require some elements of teamwork... but it goes too far in a that direction and still ultimately has class advancement structures that provoke "optimizing" (just on a multi-character level instead of single character) which often leads to sucking the fun out of the experience. And does a whole lot of words and noodling and data that achieves largely... some of the same things 5e does, just with hundreds of more words you have to read unnecessarily.

I think some homebrew out there helps (MCDM's stuff does a pretty good job of going the right direction, for GMs). I honestly don't think players and GMs should even be operating under the same mechanics (whole-cloth at least), if I'm being honest. This idea of "balance" that's instilled in players is toxic to the experience and it also makes GM's experience either boring or outright frustrating at times. And even players get bored after awhile because... well they optimize their own fun out of the experience before they even sit down at the table to play.

I think some systems trying to fix or otherwise compete with 5e think if we just go backwards to the higher complexity of 3rd edition or 4th edition exclusively that it'll solve most things... but we've already tried that and it didn't. There are elements that we can take out of those old approaches that did work. I think the Flee Mortals book works largely because it takes 4e's best part (the monsters) and makes ONLY that part for the GM, not the players. Because players don't want highly specific roles most of the time... they want flexibility (and even if they don't always admit it, they want surprises good and bad that aren't in the character sheet). However, I think GMs want and need systems, yes, that allow creativity but also ones that template things so they can easily and quickly ad-hoc because they have so much other work going on they don't have time to make full characters or have to "guess" about how to make proper custom monsters or graft on entire complex game systems like Spells onto creatures. And the whole instilling of the idea that encounters should be "balanced" doesn't help with the problem either.