This is not specific to Star Trek.. TV longevity has almost always brought with it a propensity to be typecast, or even be uncastable thereafter. You think that dude Aaron Paul, who played Pinkman on Breaking Bad is having an easy go of it, right now? People probably still be yelling "YO Bitch" at him in the street
I figure Lost ended just as many careers as modern Trek. I mean... What's Hurley up to? Even Charlie doesn't work much anymore, & he was in LoTR. I'm pretty sure that's why Terry O'Quinn didn't initially want to play John Locke, but honestly, if it hadn't been for Sawyer, no one would even know who Josh Holloway is. It's the choice you make. They pay you a LOT of money for a long time, & the trade off is that you wear it in syndication for ages after that. Everybody should know this by now. It's been a thing since syndication began back with Gilligan's Island, & the end of Alan Hale Jr's career
A celebrated tv character is a double edged sword. It gives actors a monumental notoriety, that most of them almost never would've gotten otherwise, because they're just tv actors, (Some of them crappy ones) & are relegated to the small time, but the price is they are branded by that character. Norman Reedus had a career long before TWD, but he'll be Daryl Dixon the rest of his life now, even if they finally kill the character off, & it'll be up to him to find a way around that, if he can. He'll have an easier chance of it than Andrew Lincoln. He's done after Rick Grimes, maybe. You think anyone sees Jason Alexander, & doesn't think George Costanza? C'mon
If you're going to do tv, it helps if you already have notoriety, & it helps even more if they kill you off quick, like Ed Harris & Anthony Hopkins on WestWorld (Though you may see them reprise) What harm could doing tv do to those old guys at this point? But those kids on Stranger Things? The only one who's got half a chance is the little girl, if she stays on fleek like she does
If a show is widely successful, maybe only one actor survives to work on up to bigger & better things... like Jennifer Aniston. The rest disappear, work in obscurity, struggle, catch a break now & then on something, but almost never reach that level of acclaim ever again