A soft cap is not a cap. Don't underestimate powergamers. There are enough hours in the day for them. If progression can be made, people will go after it. Trying to convince yourself that people will stop achieving gains just because it's painfully slow is naive at best. On the other hand, by you saying it would be "impossible" to do, there might as well be a hard cap. That's what your "impossible" would be achieving. A hard cap would be much more simple to code.

You don't want decay or a hard cap, yet you want gains slowing down to a point where it's impossible to max everything, but you also want to be able to keep developing your character. Makes no sense. You either have a system where gain stops at some point or one that keeps going (even if it's terribly slow).

Another thing... You don't even know if skill decay will happen until you reach a certain soft cap value. It's possible that's what he's thinking of doing. Couldn't tell ya. All he has said is soft cap with skill decay.