> If we find new particles and also come up with a theory that extends the SM to describe them, that new theory _becomes_ the standard model.
I don't know enough history to know if this was the case in the 1970s or 80s, but I strongly suspect it wouldn't play out this way now. The existing "standard model" has been so stable for so long (30 years or more) that it is now treated as a very specific thing. When theorists talk about possible expanded systems with additional particles, they tend to give them names: the "MSSM" is the "Minimal Supersymmetric Standard Model", for example, and if that proved to be an accurate description of our universe I think it would carry the MSSM label forever. I expect that even decades from now when we (hopefully) have a well-established broader model in place, the term "standard model" will still refer to the same low-energy sector of that theory that it does today.
I don't know enough history to know if this was the case in the 1970s or 80s, but I strongly suspect it wouldn't play out this way now. The existing "standard model" has been so stable for so long (30 years or more) that it is now treated as a very specific thing. When theorists talk about possible expanded systems with additional particles, they tend to give them names: the "MSSM" is the "Minimal Supersymmetric Standard Model", for example, and if that proved to be an accurate description of our universe I think it would carry the MSSM label forever. I expect that even decades from now when we (hopefully) have a well-established broader model in place, the term "standard model" will still refer to the same low-energy sector of that theory that it does today.