I agree with most of what you said there, except for this. James Doohan WAS Scotty. Saying he does Scotty better than Scotty, is like saying some actor in the future could be better at being you than you.
It's very much
not "like" that lol.
Scotty was not ever a real person. That's genuinely important to remember because clearly there's seemingly some confusion here, given your insistence he is
the same as a real person.
He was a character on a TV show
That's different from being a real person.
If I'm not real to you, just another fictional character because I'm a poster on the internet, well, I can't help you there!
But having an actual Scottish accent, rather than... whatever Doohan was doing... plus being a more capable actor (which, sorry, he clearly is imho), and just being more convincing as a "highly inventive, paradigm-shifting engineer" adds up. The first person to play a role is not always the best! (Even ignoring examples with pre-existing characters, i.e. all the Marvel, GoT, etc. examples, c.f. Becky from Roseanne, Ann from Arrested Development, Reggie from Riverdale, Carol (Ross' ex-wife) from friends, and so on.)
It's fine to prefer Doohan, I mean, some people prefer Aunt Viv 1.0 in The Fresh Prince (I am one of them), but you actually can say a new actor in the role is better at it than the originator, because the role is a role not a person. Hell with Shakespeare I am very confident there are even parts written for specific actors at the time that have been played by other actors "better" truly countless times since.