How long of a life do you see the console having?
I think it's safe to say Nintendo won the 9th gen.
Pretty much no chance the PS5 or Xbox Series X|S will outsell the Switch. Combined PS and Xbox sales have been relatively stable for the past 3 generations with combined sales of around 170 million. I'd expect Xbox to eat into PS sales this generation due to its far stronger first-party lineup than the last generation.
As for how long of a life do I see the Switch having? I wouldn't expect a Switch successor until March 2023 at the earliest. Holiday 2023 or 2024 for the Switch successor seems to be the most likely window for it to launch.
At the end of 2023 I can see Switch sales of around 135-140 million. Sales of 20-22 million this year and 15-18 million next year.