In response to the claim that I don't care about the game's lore I'd like to point out that a significant portion of my posts can be attributed to the lore forum...which is pretty much the only other forum around these parts where I lurk other than general discussion. I also frequently discuss the game's lore over Discord with other players.
Lauront has the right of it, though - there's way to make the inclusion of Pureblood Garleans work and even if it did involve some light tweaks here and there it wouldn't be any more jarring to me than some of the other potential supposedly 'outlandish' options such as male Viera.
It's an MMO. It's not going to be set in stone. The story and game is going to be tweaked and adjusted to make it more interesting over time. It's not going to constantly stay stagnant and never, ever have any twists or turns that might change things up.
It wasn't until recently that Tempering was thought to be incurable. Yet the circumstances changed as new lore came to light.
It wasn't until recently that male Viera were thought to never be present outside of their home and yet that is going to be changing as of Endwalker.
I disagree. As per the reasoning outlined in this prior post:
We already know that not every Garlean is aligned with Garlemald and that has been the case from as early as ARR. In both the MSQ's and many side quests, the day is saved precisely due to the aid of a Pureblood Garlean.
I don't see how it'd be jarring for that to extend to the player character as well given that he - or she - can already be designed to be one of many different playable races or jobs.
The defector trope is also very common in JRPG's. The cast of FFVI didn't cast out Celes because she was formerly associated with the Empire in that game.
General Beatrix wasn't cast out for formerly being loyal to Queen Brahne in FFIX.
Larsa wasn't cast out for being aligned with the Empire in FFXII - because he was ultimately working to bring about peace.
Though in fairness, I can't expect people to have played every other Final Fantasy game in the series/be familiar with their story elements.