We're talking about companies deciding which platforms to develop their games for. Will the fees/royalties for PS5/Xbox be more expensive than for Switch? I've no idea, but even if they are they won't be "much more expensive".
Besides, one also needs to take into account potential return on investment rather than just how much it costs to develop for a given platform. And on that basis... you're better off going multiplatform 99% of the time!
I'm only asking you based on this line: "why would you keep buying the more expensive one?"
It impress me that so many people in VGC ignores that companies work to maximize profit in almost 100% the situation, so yes most of times they will be multiplatform, they will avoid multiplatform only if the porting cost doesn't return enough profit to make sense (because resources are finite so even if the port makes profit not always it will be done because they can make more profit on the same resource doing something else, be it a DLC or a new game). But we have so many that think companies think like them based on hate and love.
Um, I only think about the people who researched a lot about the history of mistakes from a company which they don't even like but somehow completely ignore/forget the history of mistakes from their favorite one.