How many people play to 20 and then continue playing at 20th? How many more adventurers/ encounters are you going to have after hitting 20? Curious, I have only been in a couple campaigns that reached max level and we normally only had 1 or at most 3 sessions after hitting max (after 25+ years if playing).
How many people do PVP player arena in their games? Do you always brawl at lvl 20? DMs normally don't let this sort of thing happen in the games I have played. Only once did I do an arena where players had to fight each other and that was mid level.
As a DM, I think both of these issues are solvable, but I get in organized play it might be an issue. This isnt a video game, if its a problem you can change it. And, I think the designers are looking at it from that perspective as well. If something looks abuse able, it's up to the DM to say stop it. A lot of people don't like that answer. I use to be one of them, I am embracing change.