It would happen either way. It's more a matter of how long you can hold off until you HAVE to make the adjustment. If XIV doesn't do any crazy stat cimbs when they raise the level cap, then it probably can go about 9 years or more without needing adjustments I would think.
The way Utsusemi tanking worked in FFXI is that a Ninja would have to stack evasion, have a gear swap macro for Blade: Retsu, and rely on their elemental Ninjutsu to gain and keep hate. The reason for the evasion stacking was because misses by the enemy meant that Utsusemi shadows were not being consumed.As to using evasion as an exploit. I can understand stacking X means an evasion heavy class can do alot of broken things. Though i'm not familiar with your first example, i am familar with the second. I think its much better for us to instead of looking at it as its been used badly before, get rid of it to how could that be avoided in the future?
Personally, evasion should be a tank stat, but standalone rather than linked to the primary stats (STR, AGI, DEX, VIT, MND, INT, etc). It should ideally be the lowest scaling tank stat (Block > Defense > Evasion for PLD, Parry > Defense > Evasion for WAR), and work so that against normal mobs you can fully evade attacks, but against bosses and NMs succesful evades turn into Glancing Blows (you take part of the damage anyway, turning evasion into a mitigation stat against bosses and NMs).
Something that we also seem to be missing with regard to clear stat caps and diminishing returns is that in exchange people have a goal to work towards instead of playing guessing games.