My original point was focusing on DPS inside of a Full Party of 8. While your idea may be perfectly serviceable for a Light Party, this isn't necessarily the core issue being talked about here. Apologies if it wasn't clear!
There are several reasons why this wouldn't work in a Full Party scenario.
Immediately off the top of my head:
- This would require different variable health pools and damage values specifically for class composition. Which in of itself is more data that has to be attributed to every boss/enemy/add in the game. That is a lot of variables to rework.
- Moreover, by adding this sort of implementation, you add an additional layer of metagaming where parties and statics would be optimizing their party based on 'What combination of classes are the best DPS-vs-Enemy Health. This could alienate classes even further and make some classes completely unwanted based on this criteria.
(As an example, BLM and PCT may both be considered 'Greedy Caster DPS', and increase the Boss' Health, using your idea. But PCT is significantly more versatile and powerful. So why would you ever bring a BLM into the party over PCT?)
This isn't as simple you'd think; it's a lot more involved problem than it originally seems.
It also bleeds into 'job complexity and uniqueness vs job homogenization and balance,' but that's a whole other can of worms.