That's not exactly a big achievement, considering Hanna-Barbara was pretty much the only real company producing animation back then. Nothing against the shows personally, but their success was pretty much given to them considering how much of a lull there was over television animation in the 70s due to censors and watch-groups running the fun for everyone, plus Hanna-Barbara having a monopoly over cartoons. When you're the only major player in the game, you pretty much have it in the bag. The 80s gave way to more animation companies popping up and making series (a lot primarily based off toys or to sell toys, like G.I Joe and Teenage Mutant Ninja Turtles) but it pretty much was a big change for the animation industry. By the time the 90s came around, we have cartoons coming out of multiple companies and even networks dedicated to showing them all the time. Hanna-Barbara was being pushed aside quite easily, even though they still produced shows, they weren't the only thing in television anymore. So, you have to ask yourself, would those cartoons really earn the right to be "classic cartoons" with all the animated shows and competition they have these days? Compared to the little, if any, they had back then? So I must disagree that they "earned" it, they're classic because they were just about the only things out back then.
It's like Saturday mornings. Back then, they were popular because that's where kids watched cartoons, but now that animation has become so big that we have multiple channels airing it all day, they've become obsolete. By the end of the 90s, there were a ton of cartoons being made that there just wasn't enough room to air all the 50's - 80's stuff. As time goes on, it'll just get bigger more than likely. The more and more shows they make, the less the old ones will air because there simply isn't enough room in today's market. Just compare how many series were made in the 60s and 70s compared to the 90s and 00s (along with different companies), the gap will be immense.
So like I said, time goes on.. and with it, changes come.