I already said that not having 100% backward compatibility is a dealbreaker.
I can somewhat accept it when the physical media is different (GBA carts, Gamecube discs, PSP "carts"). When it's the same type of media (sizewise) like for example DVD/Blurays, then it's unacceptable as well.
My point is - mobile phones can do download compatibility (there are no physical carts for mobile phones afaik), so everyone else should at least do that too. For example Sony still not even allowing people to play PS1/PS2 games on PS4 is just madness. Sony got a working software emulator. There is no excuse.