I don’t think fans are particularly ‘dumb’ to just take his return at face value - no doubt, we’ll be getting a scene where Boba recounts how he survived the Saarlacc. The show is called The Mandalorian - how could the showrunners concieve such a premise without thinking “Boba could potentially be in this”. As long as it is done well, and I think his introduction was good, it’s more than just simple fan service.
Albeit I’m a huge SW fan, I would find it more henious to see something like “Mace Windu is the Jedi that Grogu conencted with through the Force” and see SLJ back in the role. Not because I don’t like the character or the actor, but that really would be a quizzical decision perhaps made for just servicing the fans.
EDIT: Or, even as I sit here thinking, including Ahsoka is more ‘pandering to the fans’ than including Boba, if anyone wanted to look through that prism. This is a show about a bounty hunter and his people, throwing in a fallen Jedi with her own agenda does/could seem a little out of place?