I thought this old article from Feb 2021 was interesting for some historical context:
RR: The levels that you can get to in Zwift, is 50 the cap? That’s the highest level right now?
EM: Yeah, it is the cap. What level are you?
RR: I’m level 40, 41, I think.
EM: Okay. It takes a long time to get from 41 to 50.
RR: I’m sure. It’s not linear, right?
EM: It gets geometrically harder.
RR: What percentage of the community is at 50? Is that like 1%?
EM: Seven-tenths of 1%. It’s quite hard to get there. Of course, once you reach level 50, it’s quite an accomplishment, because you’ve really invested a lot of your time and effort. Hopefully, you’ve been rewarded with fitness and mental benefits and social connections and all that. Don’t worry, by the time you get to level 50, there will be more levels. We’re just trying to think about a new reward structure. Our team, our designers are thinking about what that might be, that it might need to change from what it is today.
So in Feb 2021 (or so) Eric Min reckoned 0.7% of Zwifters were at the then level cap of 50.
It would be very interesting just to see the distribution of Zwifters these days (before or after new curve). Are most in the lower 25% in terms of XP? What proportion have hit the level cap of 60 as was?
How many people will take 3 or 4 years to get their XP back from the store? 
I also found this from me in a post from Nov 2021:
Joined October 2015, currently level 40.
Personally, I’m not motivated by Zwift levels,
So in 2 years I added 6 levels. And it’s still true that I’m not motivated by levels as such (I think if I did I’d ride more), but what I am demotivated by is having to re-gain XP I’ve already gained (even if I’m still gaining smaller levels).