How many Sony exclusives do you want to play in the next 12-18 months? Because that's the key thing, not the rumors about when the next generation comes out. If there are three or four games you're dying to play, a second-hand PS5 (verified, obviously) is way cheaper than waiting two years without playing anything, and besides, when the PS6 drops the PS5 will be dirt cheap.
What most people don't mention is that you can stick a bigger SSD in the current PS5 - literally just a screw and plug it in - so you're not stuck with the original storage. It's a good way to keep it going longer without dropping money on a new console.