Agreed. Anyone who has played this game online more than once knows it's not DoTA nor should it be DoTA. So stop trying to make it DoTA.
I don't get the people who claim this game isn't Dota. Yes, it is. It's basically Dota. Nearly everthing can be traced back to something in Dota. You do the same stuff as in Dota. Because it's so much like Dota is why I like the game.
Anyways...
In Dota you could also stack health and regen too. But magic resist is a specific counter. "Get more health" is a generic (and less effective) counter to not just magic damage, but physical damage as well. So it's less effective. A hard counter is more strategic and more risky (burn the gold to get greater protection from magic attacks but that won't help you vs physical stuff). I don't see what the problem would be in adding something like this. More options = better game.
Dota also had ways to increase magic damage which Demigod does not really. So it all balances out
The scepter only boosted certain ults (they add more and more nowadays though). Normal abilities you just got more mana regen to spam them more (same as in Demigod).