I was initially concerned about how the Switch would last given the fact it is another Nintendo console that is technically behind the other big 2 (yet again!). Whilst this doesn't really matter too much from a graphical perspective anymore (we haven't had a major graphical jump for years) it is a problem in that the major developers who make cross-console releases for PS and Xbox won't be able to fit their games on the Switch. They're struggling now apart from a few notable exceptions (Bethesda & Take Two in particular) so they definitely won't be able to port over new games from the next gen - at least not without major compromises.
However there is hope here. The Switch was never meant to go up against these 2 in a technical battle, it's here to supply the alternative gaming option and it's done a good job of that so far. The Switch's unique hybrid ability means it can work as their next full home console as a successor to the Wii U (this is how I believe most people see it) but it can also work as their follow up to the 3DS (how I think people will eventually see it). What I mean by this is when the next PS and Xbox come out in presumably late 2020 they will have launch prices of somewhere between £350-£450. The Switch will be a budget system at around £200 (hopefully even less) and will have a massive library by this point. A lot of people will still be buying the Switch as a cheaper portable console (I can't see Sony or Microsoft releasing a handheld for next gen). This isn't even factoring the hardware revisions Nintendo will bring out to boost its appeal (I'm thinking of an even more cost friendly "lite" version or a more powered "pro" version).
It's hard to predict but I think the Switch will last up to 2023/2024 as long as Nintendo can keep smashing it out the park with big hits and we are able to get more awesome exclusives from 3rd party developers (like Octopath Traveler & Mario + Rabbids).
In terms of total sales i'm going with an optimistic total of between 80-90 million.
