But Spock is very much defined by Nimoy's mannerism, even more so than by his character's "written" backstory. As such, trying to recast/recreate him, are much more futile than on any adapted character. Nobody will ever think of Alden Ehrenreich as the "true" Han Solo, no matter how many movies they will make with him. It can probably be done - but IMO it's just not worth if it if the alternative is creating an original character that fits better.
I don't really think that makes a big difference. Even with an "original" character, a new actor can make the character their own and be accepted by a new generation of viewers.
Look at Katee Sackhoff making Starbuck her own--and for several seasons longer than Dirk Benedict did. Or Andy Serkis taking over the role of Caesar in the PLANET OF THE APES movies. (A character who appears
nowhere in the original Pierre Boulle novel, btw.)
Plus, let's be honest, most viewers are not all that familiar with an "adapted" character's literary roots anyway. I'm going to go out on a limb here and assume that Daniel Craig was accepted as James Bond in CASINO ROYALE by millions if not billions of moviegoers who have never read the original novel by Ian Fleming. And many more got their first impression of Bond from Connery, Moore, Brosnan, or whomever they imprinted on as a kid.
Times change and so do audiences. There may well be some lifelong fans who will NEVER accept anybody but Nimoy as Spock, but, again, that doesn't apply to all of us--and that reaction is likely to fade with time.
As for the new Han Solo, I suspect he's the George Lazenby of Han Solos. Turns out getting people to accept a new Han Solo was tougher than expected, just like trying to replace Connery with Lazenby was, but history suggests that it can be done, and gets easier each time you do it. Moore supplanted Lazenby and the Bond films are still going strong, despite a few bumps along the way.
No reason that can't happen with Spock or Solo . . .
