Johnny Galeki has sustainablity. He grew up as a teenage regular on the original Roseanne series and has a fan base from way back. Maryam Bialik also has a fan base from her child star days in Blossom, and you have to give her credit for not going all Miley Cyrus on us as she has matured. In fact, she actually has a STEM PhD and is also an Orthodox Jew.
Parsons really is a good actor. He was excellent in Hidden Figures, playing a mathematical engineer in the early days of the space program; and from what I've seen, he has resisted banging the drum for gayness in his straight roles. The topic hasn't arisen on BBTheory (at least I've never heard it, tho I've watched only the last couple of years), nor on Young Sheldon.
I hope for their sakes they're all smart enough to know that, once the show ends, they'll never have this level of success again, and have invested 90% of their pay. Oh, and for our sakes, I hope they're self-centered enought that they don't spend their millions trying to influence elections.