This seems like the easy fix, but then a team has zero chance at a comeback if they are down two scores because it eliminates the onside kick.
I don't think that happens often enough to really be a huge concern. It's very cool when it does but the percentage of onside kicks that actually work has to to be pretty tiny. I imagine you could gimmick in some sort of rule to replace standard onside kicks too if you really thought about it for awhile.
I don't remember the exact number but kickoffs lead to concussions way more often than any other play, for what is ultimately a pretty minor part of the game. I don't think anyone would really notice a couple years. No one really gets mad at the concept of a touchback so it's not like the kickoff is THAT important