To be fair, this seems more likely with the 2024 rules adding the possibility of continued advancement via epic boons. I'm interested to see how that plays out. Though ultimately we are talking about probably less than 1% of characters.
Certainly with my current monk character, that capstone is a HUGE carrot. Even if we don't play at that level for long, I just want to see what it feels like playing a monk with that kind of power. First thing she does is find an ancient gold or red dragon to beat up and steal lunch money from.
The old rules had that too, I just never played it (either as a DM or as a player). Although I have a lot of experience going to level 20 in the last 2 years in all those cases I was ready for something new and there was no desire to continue on.