there's always the next wave, man. low print run gen 6 games and le hidden gems are already shooting up in price. people can speculate on gen 7 all they want but with online-only, dlc, and dying optical media, who knows what the future retro game market will look like. I always like to cite the 2600. Look at all the anthology collections that are actually on retro consoles like the ps1 and gameboy of intellivision and atari games. There are not a lot of serious collectors for those games/consoles, but they were hot shit at the time. At some point, games become inaccessible to future generations and will not command the same prices. any europoor on this board will tell you in a second that most retro game discussion is dominated by japanese and north american console games... for now. Games had huge global penetration at the end of gen 6 so it will be weird to see what other countries find desirable or nostalgic.
As for where pre 2000 game prices are going, i think you will see classic titles remain steady and less desirable stuff will drop in price. It's already happening with the nes. mega man and zelda will always have a minimum price, but something like adventure island may drop back down to $5 because it just does not have the staying power/nostalgia even though the games are great