since its possible to reset the enchant of the the hero by 80% (free)
why they dont do reset hero mercenary or even skill? 80% too?
Because that wouldn't be a good business model.
If we could transfer our resources around from hero to hero so easily we wouldn't spend as much on upgrading heroes:
-Player spends $ for resources to upgrade hero
-New hero comes out
-Players spends $ for resources to upgrade the new hero
-Another new hero comes out
If you allow all of the resources to be refundable (say at 80%) then step 3 is greatly reduced and/or disappears - so less $ spent
But even then it's thankfully not a complete waste to upgrade your heroes because almost every upgradable aspect of a hero is beneficial as an aid for your new heroes:
Upgrading skills - Better aid skills
Upgrading levels - better HP and ATK bonuses for aided heroes
Upgrading mercenaries - better HP and DMG RED for the aided mercs
The only thing you have to be careful about are talents, spells and evolution, which aren't refundable or beneficial in any way (except evolutions that increase level +5) if you decide you don't want to use that hero.