That's a valid point. Admittedly I don't know how long it takes to collect an individual spell. Is it a big time sink or is the process itself rather short?

Maybe this could be avoided by giving evolve mode only the same amount of new spells per expac other jobs get? When a new expac drops you'll have to level your regular jobs to the new level cap anyway.
And I assume, since evolve mode is all about button/skill efficiency, we will probably get fewer skills per expac than in the past.
So if you're in a raid group it would be on you as a BLU/BST to collect the skills on your path to the next level cap.
Since you only have to do this once and the level cap (and with that your number of max skills) stays the same for the next couple of years until the level cap is increased again, collecting spells shouldn't be an extra burden past the early phase of [Expac number].0?