I don't agree. As I said, those actors are far better-known in those roles to modern audiences and played them far more recently. That would make it more intrusive.
And I disagree that the roles would be minor. If Supergirl ever actually does a story in which Superman, Lois, or Lex appears onscreen rather than just being talked about, then it is going to be a huge deal and a major storyline. And the show would have the right to lay down its own pipe, to define its own continuity where those characters are concerned, just as it's done with the other characters that Smallville used, including Kara herself, Jimmy Olsen, Cat Grant, and Toyman. And bringing back actors from a show as recent and long-running as Smallville would just get in the way of that. Even if I were a bigger fan of Smallville, I'd consider that inappropriate and distracting. We've already had a decade of Smallville. It's greedy to demand more. Let Supergirl be Supergirl.