There was a guy I knew in my FFXI days who confided in me once. He told me that he'd made some poor life decisions early on, and was now in a situation in which he needed to work two menial jobs just to stay solvent, with no real prospects he could see to get himself out of the hole he'd dug himself into. He had a limited amount of money to spend each month on recreation - and what he really loved doing was playing FFXI.
Is that truly a childish attitude to have? Maybe. I don't know. Regardless, it's just one example of an individual who would need to think long and hard before investing in an upgrade. I'd imagine there are plenty of folks who'd get that thousand-yard stare once the announcement came out that they'd need to shell out a couple hundred bucks or stop playing.
It's also important to realize that a lot of folks DON'T have their own humble beginnings. A strong portion of the playerbase comes from middle- and upper-class individuals who never really wanted for anything. I'm one such individual; while my family was not rich by any means, we never had to deal with crushing debt or anything along those lines. I try to stay cognizant of just how incredibly lucky I am not to have had to deal with those kinds of problems, and never just assume that "a few hundred bucks" should be a small price to pay for something.
One factor to consider, too, is that if individuals have the means to easily upgrade to PS4, they probably would. I played on PS3 for a while, and it was, indeed, pretty miserable. Load times were awful, FATE monsters were invisible and untargetable, and larger monsters moved like old-time stop-motion animation - it was such a breath of fresh air when I moved to PS4! SE doubtless has the means to detect how many players are still playing on PS3, and my guess is that the bulk of those individuals are folks who are either reluctant to or outright can't afford to upgrade. SE has a pretty good idea of just how many subs they will lose, and until the balance of subs lost vs PS3 development costs swings in their favor, nothing will change.