So basically I have only two downloadable games on PSN: Okami HD and Journey.
And I was thinking...when the PS4 will be out, we will be able to downloade the purchased content on PS3, or Sony will take down the server?
I was thinking to buy the Collector Edition (retail) for Jorney (so I can grab the soundtrack too), but for Okami is a problem (Capcom made a retail edition for this, but only in Japan).

The PSN will exist, as Steam or GOG onc PC, or Sony will nuclearize it at some point (as Microsoft did with first XBox server...but it was a different thing, because there weren't downloadable games at that time, only a bunch of maps for Halo 2)?

From what I know (which is admittedly not much), the PSN will continue to exist. After all, Sony is well known as a company who supports their consoles even after the release of their successors. And besides that, the Vita also uses the PSN, so it would be pretty embarrassing if they shut it down. And while you might not be able to transfer your games from the PS3 on the PS4, Sony has said that they will probably use the Gaikai as a means of emulation, which is better than nothing, I suppose.