Nobody said first party games don't sell better. That is not the benchmark for success. Third parties do not need to sell as much as Nintendo's games to be successful, all they need to do is be profitable, and again, the fact they're still being brought over in the system's 4th year proves its worth doing.
It's actually not at all enough for a release on Switch to be profitable. It has to be more profitable and offer a higher return on investment than other options or ways that the time, staff, and money spent on the transition to Switch could take.