[phoenix_wright_OBJECTION!_I think almost all of the options are contributing factors, but I want to push back against this one (and apologies if this has been discussed already, I didn't read all 45 pages of the thread):
If it's taking games too long to reach high levels, it's because in my experience, DMs level their players up at far slower of a rate than the game expects. The 2014 DMG had a blurb on "Session Based Advancement" that laid out how often the game expected players to level up - and it was about once every 3-4 sessions. At that pace, it would take roughly a year and a half of weekly sessions to go from level 1 to 20. Meanwhile, I feel like I often read posts saying things like, "our campaign has been going for three years and we just hit level 8."
I'm not saying that these DMs are wrong for leveling up their players at such a slow pace - as this thread shows, there are many reasons they might want to keep their game at low levels for a long time. But I think saying the system takes too long is absolutely incorrect.

There are two serious problems with the foundations of your post. Firstly is the fact that "high level" starts well before level 20. Wotc has put out data showing that most games end by around level 10 & drops off till it jumps a bit for level 20ish one shots. Secondly is the fact that for a decade now people have complained about problems in high level gameplay that start late tier2 early tier3.
Combine the two and "high level" starts well before level 20 where it eventually finishes evolving into something else beyond high level play.