Ignoring the fact that doing this is extremely unethical to the players who have and are comfortable with their normal/rare fifth hero armor, let's talk about the two arguments I see in your proposal:
1. Normal and Rare POS don't have element resist, so why does the Fifth Hero set have it?
Equipment shouldn't be compared with each other, especially since the Fifth Hero is at a higher level/grade.
I could make the same argument between BDL and POS:
- Why does the rare BDL set have element resist when the rare POS, a set of higher level/grade, does not?
- Why are we able to add element to BDL weapons when we can't for POS weapons?
- Why does the POS set lack a shield when there's one in BDL?
Answer to all these questions is that they're two different sets and should not be compared with one another.
2. Having element attack/defense attack on normal/rare Fifth Hero give less incentive for players to upgrade their equipment.
Though I agree this is true, this system where the value/cost ratio of a stat decreases exponentially with higher quality items can be seen in many areas of the game. However, there was never any complaints for any of them. Here are some examples:
- Skill cards: Upgrading a skill from Lv.9 to Lv.10 cost almost twice as much as upgrading a skill from Lv.5 to Lv.9, but the value of upgrading from Lv.9 to Lv.10 is significantly less than upgrading from Lv.5 to Lv.9.
- Socket cards: Upgrading from 8* to 9* is less cost effective than upgrading a lower * card with the same amount of plasmas.
- Soulcraft: Soulcrafting an equipment from normal to advanced gives ~50 soulforce. Soulcrafting an equipment from artifact cost like 10x more and only increase soulforce by ~10.
So why do players still do these things even though they aren't as cost-effective? Because some players like to grind and maximize their gear. If you are happy with normal/rare Fifth, good on you! I know many players who are satisfied with their normal/rare Fifth. Heck, I knew many people who were satisfied with normal/rare POS sets before Arcadia 3.