Er. I don't think they'll discontinue either system or the servers for XBL/PSN for previous-gen systems right when the new systems come out. You have to understand that people who don't post on this forum or other gaming fora may not be so quick or readily able to adopt new platforms right when they come out, or even two years after they are released.
In fact,
didn't MS once say that they'd like the 360 to coexist with the Xbox One at one point? Yeah, they're not going anywhere for the next two years. I don't think Sony's going to drop PS3 support, either. Even the
PSP is still getting support in Japan.
It honestly does take a while for systems to be phased out. People were still playing PS1s a few years after the PS2 came out. People were playing SNES games a few years after the N64 came out. You could still even purchase those platforms even after the new platform was released.
Not to mention that there are markets that would still be more prone to purchasing stuff on the Xbox 360/PS3 simply because the PS4/Xbox One may be more expensive, they may not have access to those platforms in those territories, or they may not have an abundance of those systems in those territories.
What you're saying may be relevant to us on this forum and other games forums, but it certainly may not be relevant to others who won't invest in a new system as of yet even into 2015 (ex: those who don't play games as often yet still purchase things digitally, those who would wish to stick to their older platforms longer, etc.).