I agree that there isn't a need for a new system but I don't think they'll sell poorly. Eventually they'll stop making new games for PS3 and I'll have no choice but to buy PS4.
Twenty years ago my dad said "why would we buy a new Nintendo, we have a hundred games for the one we have". A year later they released a new Zelda for SNES and he bought a SNES.
11/30 -- Devin Townsend
12/06 -- Distant Worlds: the Music of Final Fantasy
12/27 -- Unearth
03/07 -- Video Games Live