I think you have that backwards. "junk" is an insulting word. If you called an Atari junk, most would understand you to be disparaging it in a roundabout way, not making a temporal point. To be fair to OP he did at least call it "ancient junk." But there's no need to add "junk." "old" or "ancient" more than ably would have made the point.
Let's take some non-human examples for comparison. Just about every good military strategy produced when your grandpa was young is, objectively, junk when compared to the state of war today. But just about every good piece of music still holds up - it might be different from good music produced today, but it's not worse. Your grandpa probably did mathematics with the help of books of log tables and books of mathematical instruction. The former are thoroughly junk, being at best as useful as a calculator (or calculator app) but much heavier, and at worst bearing typos; the latter are every bit as useful now as they were back then.
Describing something from a previous generation as "junk" does not convey the belief it is old - there's a perfectly good term for that, "old." It conveys the belief that it is junk.
So being pedantic, it is optimal in this specific meaning, but I don’t know about this stagnation.
By now, it certainly is. Back then it was probably state of the art. That's the cycle of computing. If I called an original Atari junk, some would take it as "this is useless" but most would take it as "this is useless now".