My stance has changed on this. I don't think GameCube will be added to NSO like the apps currently exist, but I think GCN and Wii titles will see more and more "quick and dirty" ports like Pikmin 1+2, and that these will be added to NSO+Expansion Pack around the launch of Next Gen.
This approach gets around issues like adapting controls and large download sizes, while people who want to "buy to own" still can.
I think DS is the final NSO app, probably next year, with additions after that being Wii and GCN remasters, then early Switch games, transitioning into a "Game Pass for Nintendo".
They can't keep adding consoles forever, because eventually they'd catch up to current gen and reinvent Game Pass anyway. I think that'll start with GCN titles. Also gives them a place to put the already ready Mario Galaxy HD and Sunshine HD ports back on sale, along with games like Twilight Princess and Windwaker HD, and the already-ready-but-never-released New Super Mario Bros. Wii.
My new feature prediction for NSO next gen would be "remote play together", giving games on Nintendo Switch with limited or no Online full Online support, just as the NSO apps do for their games, but at a system level for all games.