Actually, wouldn't it incentivize it?
After all, if you aren't getting much from your main class, its best if you grabbed stuff from other places.
Or it could just be WotC being bad at writing high level abilities again.
Emphasis on "simple". In particular, if numbers aren't a simple linear function of level in a class, they'll be quick to realize they can't have you just sum them across your classes, which is what made it so simple in 3E. There's no way in hell they'll trust the players to do square roots or logarithms or whatever they base their progressions on (if they base it on anything numerical, as opposed to "by feel"), which means they'll either have an absurdly elaborate system of table lookups, or else you'll have fixed points in your progression where you can multiclass, a la 4E. Either way, classes become significantly less modular and the system becomes less likely of being capable of representing your character idea.
They could easily miss it if gains
accelerated as level increased (they sure did with spellcasting in 3E), but I would bet US dollars to Zimbabwe dollars that they'll zoom in on anything that makes multiclassing fundamentally and strictly better than singleclassing, and rule against it.
I think they may be running up against the fundamental limitations of the d20 as a system, unfortunately, and you can't ditch it as the basis of your RNG without losing what's pretty much the only mechanical constant that defines D&D. You can't support open-ended character growth in pure numbers without an RNG that grows to match, and you can't write an open-ended progression of new
abilities at all. At some point, if you hard-capped numbers, you'd have to essentially write Epic Spellcasting again. So all you can do is slow growth down as you grow, in the hopes that nobody will ever actually play to the point where the system breaks down.
An alternative might be to redefine what a given result on a d20 means by what your level is (so that a 10 for a level 1 Fighter is a solid blow and a 10 for a level 20 Fighter is cutting down an entire battlefield of ordinary soldiers with a single swing), but that would require such a fundamental overhaul of the system that I can't even begin to imagine what it would look like, and it'd have a lot of DM Fiat problems.
Big ol stream-of-consciousness post is rambly.