The fact that there would still be a server around it predominantly focused on minmaxed level 30 builds getting a decent challenge, and that most classes have been reworked to fit into that mindset as well, for one.
The equal fact that you would be level 20(s, assuming there'd be more than one person) involved in events that are scaled to Level 30s if you end up with DM involvement, which also includes -completely souped up mobs- that are made with that semi-canon 2/3 or 1/2 'rule' in mind to represent threat, meaning you'd be a 10 to 13-and-change level way out of your league instead of legit level 20.
The simple issue is that we have no clear reasoning behind why we have 20-60 Demigods running around BG that can reduce the City to bloody rubble in a heartbeat and the various horrible inconsistencies that causes mechanically and logically, together with the ludicrous level the guards have to be considered a legitimate threat.
So ultimately, the Level 20-cap is meant to make the world make sense again, and as examples above have noted, it'd not even necessarily downgrade the total power level by much while making mobs way easier to handle because you don't have to adjust everything with them that hard anymore.
I do get the concern with having to rework areas and all, given the light population of the dev team however. So I am aware that is a tall desire to fulfill. Can't help but dream, away from the server as I am now... =)

The Fire Giants, with the Balor provide a similar challenge at max levels that the Gibberlings and Gibberling Chieftain do at level 3 to 5.