Here is my input. The term "classic" is only a title and should not matter.
Classics Shouldn't Matter
Introduction
When we hear the term "classic anime" we usually recall titles such as Evangelion, Cowboy Bebop, Fullmetal Alchemist: Brotherhood. However, we all know that the anime industry is far more diverse than that.
The otaku community has developed so much that we have more titles now: Land of the Lustrous - "hidden gem"; Naruto - "king of shounen"; Attack on Titan - "modern classic"; Your Lie in April - "tear-jerker"; Puella Magi Madoka Magica - "literally the only dark magical girl anime that works". These animes earned their title through spectacular animations and terrific writing, and yet we all know that titles are not everything. Let's roll back a few seasons to Bunny Girl Senpai. I'm confident most people will claim that this anime is extremely superb, but I haven't heard a title being awarded to this show. How about isekai - the most oversaturated genre in anime? We all know horrible examples
cough Conception cough In Another World With My Smartphone cough Master of Ragnarok cough but out of a hot, steaming pile of crap, diamonds can be found! KonoSuba, Overlord, No Game No Life, Shield Hero, etc, etc, etc. What do we call them, "good isekai"? I haven't heard such a title before. And what about SAO, pretty much one of the most popular yet hated anime out there - why are so many people calling it trash meanwhile its fanbase is so enormous and still growing? I don't think anyone's gonna call SAO a "modern classic".
Why it shouldn't matter
I'm gonna use one of Gigguk's conclusion from one of his videos here. People love classic animes in the past because the community was so small and not that much anime were released. Classic animes were used as guidelines to sliver into the tiny anime community back then. Since it was so miniscule, a lot of people has similar tastes thus why there are a lot of old anime that are, in a way, space operas.
And nowadays, this community has grown so big. Slice-of-life, action, fantasy, adventure, sci-fi, comedy, romance, isekai, shounen, shoujo, harem, yuri, yaoi,
imouto ecchi... There are so many different genres one could enjoy within this medium that an idol fanatic may find Steins;Gate sleep-inducing or perhaps an ecchi-addict will watch shounen only to fap to the fanservice scenes while skipping all the action.
Such guidelines are no longer necessary because the wide range of options to pick from causes it to become obsolete. You no longer need to fit into the anime community, you now are able to search for a subcommunity within it that you automatically fit in.
"Classic anime" is a title awarded ubiquitously by the community; like an Oscar to a singer or a Nobel prize to a scientist. They have served their time as guidelines and have now retired alike other titles mentioned previously. The modern community is able to view a "classic anime" just as it would view a "good isekai", a "quality shounen", or literally whatever you can come up with. These titles are trophies served to show their great achievements to the brand new eyes. A newcomer would be attracted to a "hidden gem" because of said title which the community has given to the anime.
And heck, SAO doesn't have any of the above titles that I think it created its own unique title: "that one poorly-written anime that literally all animenoobs watch first". If SAO can do it, I'm pretty sure any other anime can. SAO is proof that you don't need to earn a title to be successful.
Conclusion
Anime classics, at the end of the day, are classics. But "classic anime" is a title alike other titles. Titles are trophies awarded to the anime by the seemingly unanimous decision of the community as a whole. These trophies are clear signs to a new pair of eyes that the anime in question is of high quality, however, that is all. The anime community today is so large and varied that any anime would have its own targeted demographic and can be assured to fly under their wings; an anime does not require a title to survive in the modern era. Therefore, "anime classics" do not matter.