Why HP inflation is a valid concern

[Kitfox]

Why would we need to alter incoming damage or VIT for other classes when literally the only thing that changes is if tanks get AP from VIT or not.

Because if VIT increases both AP and HP, you can't increase/decrease one without affecting the other. If you want to change AP but retain the same eHP, you would need to alter incoming damage to match the HP change, otherwise tanks would have relatively inflated HP on AP increase and deflated HP on AP decrease. Now, if you alter incoming damage, you also have to adjust tank def/mdef or VIT (def/mdef) for all other classes, because raid AoEs hit everyone, not just tanks, unless you don't care how much they hit tanks for. For any AoE mechanics that are preceded or followed by tank busters, you probably would care.

Alternatively you could adjust VIT:AP multiplier every time you want to make a change, which would affect the whole level range, unless they have level specific modifiers for the value. That could possibly break old content tuning, which you might or might not care about. Old players probably wouldn't, new players might.