Why do almost all editions of D&D fall apart around 10th level

[BayardSPSR]

In AD&D, there was a specific intent that the play style would change from individually going out to solve problems below 10th level into ruling groups and bringing comrades along above 10th level. There was a reason that most characters got followers around that time, and why HP switched from dice rolls into flat +2HP/level at that point.

...

But yes, the spell lists are causing a big part of the problem. Everybody is so focused on reproducing earlier editions with their spell lists that they don't focus on which spells are good to keep and which ones can be avoided or dropped - or, for that matter, how to change casting and spell designs to fit a particular theme.

It sounds like later edition designers confused "reproducing D&D" with "reproducing D&D."