I feel like the reveal was intentionally ambiguous and the revelation was kept somewhat open-ended. The Hex hasn't been lifted yet when Monica identifies him as Ralph Bohner. So while this could mean that Agatha's runes affect the whole house rather than just the basement, Ralph could have just been his first WandaVision persona before Agatha used him as the Fake Pietro (especially since she previously named Ralph as her husband).

They may not have immediate plans for him, but I imagine Marvel were thinking of other, non-Quicksilver roles that Peters could play later in the MCU, so they used him here as Fake Pietro for the in-joke but still gave themselves freedom to possibly bring him back. If he never does, we can just accept that he really was just Ralph Bohner and think nothing more of it, but if they cast him as a different character later (Simon Williams, for instance) they can easily retcon that that Ralph Bohner was a fake name/persona and he actually was his new character all along.

If they ever did do a multiverse crossover with the old X-Men films, they could even say that he really was an alternate Pietro that was trapped in the MCU, but I doubt they'll do that. Marvel will want to move on from that franchise, and Peters' past connection to the character of Quicksilver doesn't need to be anything more than this in-joke.