Nintendo exclusives will be the biggest games on the system, but the top sellers don't represent the whole ecoystem. This graph should help explain.
Increasing third party support decreased Nintnedo's dominance, but still enabled high sales for both first parties and third party. And these numbers don't include indie / digital only games.
Basically you can have third party games that sell very well, even if they don't hit 20 million on a single platform like Nintendo ones can.
It will be the same with Switch 2, both third party and first party will sell well.