Well it’s pretty easy actually. We know that they already designed the costumes and probably put a lot of work in that, so they want to realeasy what they probably think would be a good idea.
To do that for event, S2 and seasonal heroes that are already borderline op and not make everybody who already own the heroes feel bad/betrayed just remove the costume bonus.
I think that would be OK. Of course the costume version is better in some versions like killhare where you get 350% dmg instead of 300% but that can still be changed. So that the costume version is not better but different.
If you just give all the already good heroes a different costume and not the 5% Attack and Def, 10% health and Mana Bonus, there is no big problem. You don’t have to pull again for your Finley or Jabberwock where you probably did 100+ pulls for. They are still as good as they where befor and getting the costume would only give you another hero. So nobody would feel betrayed or like their heroes got" nerfed" because they are now missing the bonus.
Btw. Leveling up heroes beyond lvl 80 is just a bad idea. If this will be a new ascension with new ascension materials I feel like all hope is lost for this game…