The fact you can't play PS3 games on it should've been the biggest, most obvious reason you can't replace a PS3 with a PS4.
... And admittedly yeah,t here is patching in the other stuff, even MP3 playing. Hopefully they do that eventually.
I have about a terabyte of DVD rips (I own all of the physical discs, thankyouverymuch) and 60+ gigs of MP3s that I legally own, and being able to play them has been one of the primary uses of my PS3.
I was hoping that the PS4 would do what the PS3 did, but with a better interface, so that I could just fire up the PS3 if I felt like playing an old game.
Right now, it seems that I'll be better off just keeping the PS3 as my TV-connected device, and I'll look into getting a PS4 if there's a game that just seizes my attention.
In the meantime, I'll just keep using the PS3 for Netflix and Hulu and for playing all of the kids' movies I bought for my daughter.
Maybe next year, Sony.