You know what? It's a great game. It's probably my fav version to date: 4e ended up being just too cumbersome for us; 3e had the exaggerated gulf between casters and non-casters, 2e was a little too all over the place (freelance writers with little oversight played a huge part, I'm sure), 1e was...well, it had all the shortcomings of being one of the first RPGs.
It is by no means perfect. Because people learn from previous experiences, I expect each version to be better than the last. 5e improves upon a lot of things, for sure. In some areas, it seems to have inexplicably taken a step backwards, and I genuinely don't know why.
Yep, 5ed is a great game. Fully agree on that and yep it did take a step backward.
When did having all the info needed to run a monster in one place become a bad thing? When did having suggested encounter groups for said monster included in its write-up become a bad thing? When did giving it a role, for suggested combat purposes, become a bad thing? These aforementioned things only made the game easier (and therefore more fun) to run...why were they jettisoned?
In a lot of ways, the 4e MM sucked, especially the lack of lore for the monsters. New creatures were introduced with no narrative to them, making them more difficult to put in a story than necessary. But running them in the game? The 4e MM (and the tight math around encounter building) made running the game much easier.
That is exactly where 4ed failed. Too much detail not enough flavor. The videogame approach was not what people/rpgers wanted. The encounter system of 4ed had some of the failures of 5ed with big groups of 6. At least that is my experience. Maybe some other had en entire different experience than mine.
So, I don't understand the step backwards. Of the things I listed above, none of them would make negative the experience of anyone playing 5e right now if they were included. It was a solid advance made in the evolution of the game, and then oddly disappeared.
That's what's frustrating about the 'apologists': People critique the game because they want a better game, and they feel 5e might be able to deliver. Otherwise, we're left with "5e delivers as long as you have 6-8 encounters per day, otherwise, all bets are off." I can't believe that's true. "5e breaks down after level 15." Why?! With decades of experience, no one can make those levels work? Really? It's impossible? I can't believe that either. And if it ends up being true (note, I don't think it needs be true), then it's not right that the designers present the game pretending that it works fine. :/
Ok, time to go point by point...
1) The game by itself fails at level 15 because of the sheer numbers of possibilities that the players have in their hands. With all the divinations and transportation modes available you can no longuer build adventures as you used to do. The big baddy inside the tower near the top will be met first by such high level group. Either by flying or simple teleportation/plane hopping. The adventure pressure points are no longuer How to get there and Why did it happened? It is most likely that you, as a DM, will have to do the How to slow their progression, why would they go there and how will the main villain stop the players. You will have to apply time pressure very often to force players into acting without over planing with divinations and all the yaddi yadda that will come with divinations.
2) The game assumes 4 players. With four players in play, the game is much more stable. Unfortunately, it becomes swingy with 5 and 6 players.
3) The encounter build up does not (seem to) take feats and multiclassing into account. If you are using both, chances are that your job as DM is quite harder. The CR of the monsters will usualy be too low to give a real threath to players such as mine. Again the higher the level, the harder it gets.
4) If you do not enforce the 6-8 encounters per day, the players will simply go nova and do the 5mwd trick. Combat was too hard, we need to rest... I personaly use encounter tables to make sure that their sleep is disturbed until they get that amount. If they are in the vilain's compound/complex or whatever. I make sure that patrols are upgraded and they search for the group.
5) The game start to fail at 15 not only because of that but also because many DM lack the experience to handle such powerful groups. We did voice our concerns during Next play test in the surveys but it seems we were ignored. (but at least we were heard

)
6) Monster's AC is relatively low in MM. If your players are using feats and multi-classing. I strongly recomand to add ASI and feats to monsters (1 ASI/feat for each 4 CR beyond the first four.) or change their equipement. That is more work for the DM but it also adds a bit of randomness that the players that have memorized MM won't be able to factor in and it will suprise them. By doing that, all monsters become much more on par with them.
7) The worst failure of 5ed is also its biggest advantage. Common wording can sometimes leads to heated discussions at the table (and in the forums) but in the end, 5ed relies entirely on the DM's call. As long as that call is consistent, you will have no problems.
I really enjoy 5ed. It has its shortcomings but overall it is the edition they made so far. And with a bit of work, it becomes easy to let your players rise to level 20.