The BluRay drive doesn't have SHIT to do with it.
PS3= cell processor
PS4= x86 processor
A game on a PS3 disc is coded to ONLY run on a PS3 Cell processor.
A PS3 downloaded game is coded to only work on a PS3 Cell processor.
In order for a game coded for a cell processor to run on an PS4 x86 processor, it has to be recoded. This means all of the data on the disc has to basically be translated into a different computer language. Manually. It's not the same as making it from scratch, but it ain't no weekend project either.
The only reason the first PS3's were backwards compatible was that they had 2 different processors, 1 for PS3 and 1 for PS2.
So why don't they just put both processors in the PS4?
1. It costs too much. Sure the Cell is only about $50 but about half of the PS3 motherboard has to go in there with it, along with a bigger power supply and a big ass fan to cool it.
God forbid you want it to play PS2 games.
2. Remember, Sony sells every new console's 1st gen at a loss and counts on licensing fees to make their money until the manufacturing costs come down.
3. None of the new PS4 features would work when playing PS3 games.
4. Remember how a lot of the first PS2 games were actually cd instead of dvd? Cheap developers would pull the same bullshit again, selling PS3 games as if they were PS4 just because they would run on the machine. It's legal and costs the developer less to license PS3 games that it would for the PS4 game.