I'm feeling pretty confident about full PS2 emulation including disc support.
Sony have a good track record of promoting a "buy once, use on many devices" philosophy.
They introduced mainstream backwards compatibility with PS2 and retained that for PS3. Ditto to a degree with PSP.
They introduced cross-buy, which is still maintained to this day.
They've provided "free" PS4 versions of PS3 PSN titles (Flower, Flow and Pixel Junk Shooter all come to mind) to those who bought the PS3 versions.
Finally, they were quick to show two fingers to MS's support of blocking used games.
The potential revenue lost by enabling disc support would be negligible IMO. Hell, if anything providing disc support might even promote more sales. I know after some time I'd probably get sick of disc-swapping so if Sony have left me feeling good about PS2 content, I'm more likely to reward (Gradius V on my HDD? Yes please!) them with a double-dip. How many PS4 owners are really gonna have a substantial PS2 library? Not many I'd wager.
I believe at a minimum they'll offer something akin to the Xbone/360 solution as I'm not sure Sony want to lose that PR battle. But I'm hopeful they'll deliver the full hit.
However, the fart in the car is the publishers. If anyone's gonna cock-block this, it'd be the pubs I reckon.
On a final note, I wish this "Sony love to resell you content" shit would die already. There's plenty of evidence against Sony having that attitude. There's like, what? Less than 5% of the total PS ecosystem available to "rebuy" via PSN?