Retro game prices have risen with each generation that enjoyed them between the ages of 5 and 16 getting to that period in life where they have a career and disposable income without major responsibilities like kids, mortgages, marriage, and all that stuff. With that said, Gen 5 stuff was the most recent console gen to see an explosion in popularity and price, with gen 6 starting to get hit as well. I think gen 5 will continue to rise a little, whle gen 6 stuff, particularly Gamecube and PS2 will continue to go way, way up. I think Gen 3 (NES, Master System), and Gen 4 (SNES, Genesis) will continue to decline overall over the next few years since those consoles saw their prime collecting days 4 or 5 years ago.
I'm really curious to see what happens once people that were in that age bracket who grew up with consoles like the Wii and 360, come of age more. This has already happened and I feel like I've seen a very mild interest in Wii collecting over the last year, but winder if it'll really take off in the same way it did for all previous Nintendo gens. I almost think Gen 7 might be the last console gen anyone gives a crap about collecting for since things really changed during and after that gen. I guess we'll see.
But in short, here are my 2021 predictions:
1. Gen 5 will go up slightly, Gen 6 will go up a lot
2. Gen 3 and Gen 4 will go down slightly
3. Wii prices will go up very slightly, but the PS3 and 360 will likely decline some more over the next few years until people get sentimental about them.