Yeah about the subject matter, for me it isn't even just the fact that there are so many copies that are being made of the same concept that just makes it hard for me to watch way too many modern anime, it's the fact that they don't understand what makes the original good, so they basically only copy the premise/the surface level and completely ditch the ways that they use the premise in the first place, basically removing any kind of depth related to the setting.
Dungeon Meshi for example isn't great because it's an anime about people eating monsters. It's great because of how the world building is done: the biology of monsters, the ecosystems that are maintained because of the different monsters being around, and even the different relationships between the characters and what draws them to the dungeon in the first place.
It's similar to how OP characters are done right vs wrong. They see OP characters and it almost looks like people think that the fact that they're OP is the sole reason that people watch it. The modern good anime with overpowered characters aren't great because of the fact that they're overpowered and have no struggles because of it, but because their struggles are due to them being op. Saitama is great as an overpowered character because it shows that the moment he reached the peak and wasn't really challenged by anything anymore, he felt empty and has been trying to find something that made him feel anything. Mob Psycho is about someone who tries his absolute best to not consider himself superior to everyone else because he's powerful, and trying to overcome his fear of his own powers. Or for a more recent anime, Helck is about the fact that despite seeming all-powerful, he still has weaknesses and has to deal with the fact that despite being that powerful, it wasn't enough to save the people close to him, and has to struggle with earning people's trust because they fear his power.
The issue isn't the copying, it's the lack of depth.