If FF15 sells 60% of FF13 on ps3 than you could argue a large chunk of people who bought a ps3 won't buy a ps4.,
But that's where FF7 comes in. It's the most loved traditional FF and I think this remake will pull those fans in regardless just at a later date. And maybe even more who didn't buy FF13 to begin with.
That's why I think ps4 will have a good future with megatons like FF7 and GTA 6 down the road.