I still remember the massive shift to SDRP and HDRP.
Honestly, now in retrospect, almost a decade later, I think it was clearly done wrong. It was a mess, and switching over was a multi-week procedure for anything more than a hello world program, and what you got in return wasn’t something that looked better, just something that had the potential to.
Similar story with the whole networking stack. I haven’t used Unity in years now after it being my main work environment for years, but the sour taste it left in my mouth by moving everything that worked in the engine into plugins that barely worked will forever remain there.
Don't forget that some of the new features are mutually incompatible. For example couple years ago you couldn't use the "new ui system" with the "new input system" even when both were advertised as ready/almost ready
