There's nothing that prevents the creators from making their own thing that people could start from. It doesn't need to be Mario.

Game designers could also incorporate parts of "Super Mario Brothers" as a "mini game" within modern video games.

This also seems odd to me.

Generally I think I stand for reasonable copyright terms, but on the other side, I never applied those terms to Mario. I can't imagine a character like Mario being used by many companies in many games across many systems where it is not tightly controlled by Nintendo. That is really a dreadful outcome. Nintendo has grown the brand and character over the years and it would be immediately diluted.

I could very easily picture a game like GTA starring Mario using a gun to shoot goombas and koopas, while Bowser snorts crack off of Princess Peach. That's not the kind of thing I'd like to see.