It does seem that even though anime is more popular than ever, there is not so many modern anime classics.
Just off the top of my head, since 2020-ish, things that'll almost definitely be remembered at least among enthusiasts:
Oddtaxi feels like Satoshi Kon reborn, maybe with some Yuasa on the side
Summertime Render is basically Higurashi x Shiki
Bocchi is powerfully in the tradition of Watamote/NHK
Eizouken fits right in with the other Yuasa shows
Kakushigoto is classic Kumeta and will be remembered alongside Joshiraku/SZS
Chihayafuru was already classic alt-sports, so S3 is more of the same
Dorohedoro was already a classic manga and the adaptation so far has been surprisingly soulful
Jahy and Machikazo Mazoku are up there in comedic SoL with YrYr/Yuyushiki/etc
Ousama Ranking is so thoroughly well-produced I have a hard time seeing it not being remembered
Akiba Maid War and Paripi Koumei were sufficiently novel to be inherently memorable
At most only because there's so much more out there.
The general consensus is that Oshi no Ko is as good as any classic (and is by the same author as the already classic Kaguya-sama) and that only aired one episode a week ago so far. A new season of Demon Slayer started ten days ago and that's the manga with the most per-volume sales ever, surpassing even Dragon Ball. Maybe I'm wrong, but I'm talking about things that happened within the past two weeks alone.
Isn't "modern anime classics" a bit of a oxymoron? Things generally don't become classics until a while after. Lots of examples of things not really being in the mind of the mainstream until it long died.
Are you comparing to 1995-2005 or are you comparing to the lifetime of anime from about 1970 onwards?
1) I'm not sure anime is all that popular anymore. For example, I see FAR less anime merch than I did back in the early 2000s. Cyberpunk:Edgerunners is one of the few recently that seemed to spawn merch.
2) It doesn't help that a cour is now 13 episodes, at best, by default. It's pretty hard to make a "classic" anime in just 13 episodes on a super tight deadline. You can see the drop in quality around 2005 right about when things are switching to 13 episodes. By the time you reach 2010, practically nothing that isn't promoting toys is longer than 13 episodes.
3) Up through about the mid-90s, anime was mostly meant to promote buying toys and the quality reflects this (really--take a look at the anime released from about 1980-1990).
4) There were never that many "classic" anime after you remove Studio Ghibli.
5) 1995-2005 just had a remarkable run of producing some stupidly good anime along with all the standard dreck. It was a combination of decent enough funding, decent enough technology, and a sudden market opening (US demand).
6) The Japanese anime business leaders were complete and total dolts for a very long time. After about 2005, they sold increasingly expensive box sets to their whales and that completely warped things. Consquently, you get increasing amounts of junk hyper-tailored to the anime faithful (See: Zero no Tsukaima getting 48 episodes(!)--actually, please don't see it, it's total garbage).