I've never understood the rationale for the game having what amounts to an overall level cap; whether it be 20 (3e, 5e) or 30 (4e).
Why not make it open-ended, but with advancement greatly slowing after about 15th (instead of speeding up - a truly baffling decision by the 5e designers!) such that if a campaign does manage to keep going for years or even decades the system can still handle it. Better yet, greatly slow advancement down across the board such that, if desired, two or more APs can be run back-to-back in the same campaign using the same PCs; note this would require releasing a few APs that start in the 7th-12th range and go from there.