Not that big. Felicity Jones was probably the biggest.
Can't recall any blockbuster movies before Star Wars any of them were in. Not as a leads anyway.
I'm sorry, but if you think Felicity Jones was the biggest actor involved with the Disney Star Wars movies, your perspective is completely skewed, to the point of being deeply unhelpful to this argument.
Both Oscar Isaac and Adam Driver were much better-known "up and coming" actors, and you seem to discounting Harrison Ford, Carrie Fisher and Mark Hammill entirely, which is pretty wild.
As for "blockbuster movies" - with young actors that's usually the case - it's a very tiny subset of young actors that are in multiple blockbusters. So that's not really relevant or helpful.
Also, I’d say the Disney films have generally cast unknowns in the lead roles…
Daisy Ridley
John Boyega
Kelly Marie Tran
Adam Driver
Felicity Jones
Diego Luna
Alden Ehrenreich
Sorry, but this is just inaccurate. Only Ridley and Tran were "unknowns" from that list, in the Hollywood sense of "unknown", which appears to be how you're using the term. Even having been in smaller or indie movies, but ones which were critically acclaimed and which were actually shown in cinemas means you're not an "unknown". It seems the logic you're applying is "unless you were in a blockbuster in a major role you're an 'unknown'", which is... not a good usage of the term.
Up-and-coming would be accurate for most of those (Diego Luna is a highly experienced actor and was even then, just not in English-language, so it would be fair to say "up and coming in Hollywood").
I have a hard time with mainstream actors being cast in Star Wars movies, I have trouble suspending disbelief to see the character as opposed to the actor.
I'm sure that's true for you but I think it's fair to say that probably isn't true for the vast majority of people who watch movies. So I'm not sure that it's particularly relevant, and insulting him for an exceptionally good performance in a relatively good movie is truly silly.