Bear in mind that even if a new console is potentially in the works (I believe I remember reading a headline earlier about some design documents being leaked, but I didn't click on anything to see the details), we still have no idea exactly how backwards compatibility will work for it. Even if it does support backwards compatibility, it could still be... uh... terrible, due to poor emulation, incompatible hardware, and a number of other factors. So it could still be worth it to get a second unit if you really want another one. But unless you have infinite money to toss around, I'd advise you to take every avenue you can to save on it. Buy secondhand, don't go for the most expensive models, go for the lowest asking price.
Even if things were currently perfect, the main three are going to be pushing out new consoles routinely because they rely on that revenue. The main reason it appears at a glance that the Switch is exempt from this is only because this current console cycle has been particularly odd due to the pandemic and the chip shortage. As a result, The PlayStation 4 is still getting games today, despite the PlayStation 5 having been out for almost three years at this point. This is likely the situation that Nintendo is trying to avoid before they unveil any new hardware.