It’s hard to say how this will affect the second hand market. I imagine the intel MacBook Pro’s might not hold their second hand value very well when the ARM units launch/are close to launch. That said, I’m going to buy a 16 inch with 5600M and plan on keeping it for 4-5 years. I like the MacOS for productivity but like to game. There’s no indication that windows virtualisation is going to work on ARMs yet, so I don’t fancy taking the risk.
Is the computer you’re looking at good enough for you spec wise for the next 3-5 years/however long you normally keep your computer? If yes then I would buy it. The ARM announcement doesn’t make it any less good than it was before the keynote. Worst case scenario I think is that you might not get MacOS updates in 3 years time but I think they’ll support it longer. As for other software going out of date, I don’t think that’s likely given the universal app build solution covering both ARM and x86.
Might you get more for your money with an ARM MacBook Pro 16 if you wait 2 years? Almost certainly more power per watt yes. Might you have compatibility issues if you need to run x86 windows software, like non MacOS compatible games or other windows software? I think this is quite likely.