The complaint was about dipping not multiclassing. They could prevent dipping by forcing multiclassers to keep their levels equal, or even go back to their roots of XP leveling getting split between the multiclasses.only way to make dnd without multiclassing is to make classless system, which essentially 4E was.
and you need some numbers to describe that you have gotten better with levels
in weapon attacks it means better attack roll, better damage roll, more attacks, more attack riders.
preferably all 4 of those.
There's enough of a legacy for multiclassing that it's probably a sacred cow, but they could certainly make it very different to what it is today of almost complete freedom.

