Yeah I'm totally with you on this. I'm wondering if WoTC wouldn't want an expansion because they know 5e high level play is borked. The things Larian would have to do to "fix" it would probably be substantial.
There were great ways to get around a lot of it though. Did you know Mustard Jellies had a 100% Magic Resistance, and you could polymorph into one? Imagine being a high level wizard and some guy turns into yellow slime and beats the shit out of you while you helplessly throw spells at him?
Imagine being a mighty Shadow Dragon and getting laid low by a pile of animated goop.
Is 5E tabletop borked in high levels too? Are there many reasons for this?