PSN games not backwards compatible, thoughts?

I can understand PS3 games won't work on the PS4 because of the major differences in architecture and what not, but I was really surprised that PSN games won't be compatible with the PS4. I thought it was given that all our PSN purchases won't go away in vain when we move to the next console.

Now this worries me looking at this way, how worthy is my investment into these games? I mean I buy several games on PSN, and years down the road when my PS3 dies and PS3 stops being produced, I am basically locked away from all those games I am purchased. How hard is it to emulate those PSN games on the PS4 hardware, I can understand retail games perhaps taxing the system too much to able to emulated properly, but surely most PSN emulated games shouldn't give the system much of a workout right?

I really hope Sony lays a groundwork so that this sort of thing doesn't happen on PS5, we gamers invest a lot of time and money on videogames, it is just sad and unfair to loose all of them just because of some technical barriers.