Ignoring the argument about whether or not a cash shop hurts a game's integrity, what you are suggesting is not how businesses measure performance. I think this is where your confusion comes in. Businesses are not in the business (lol) of maintaining status quo. Its quite the opposite. Businesses are about growth of revenues, sales, and profits over a significant period of time. "Growth" is the name of the game...not status quo.
A sub model fails to do the above for obvious reasons. The only source of growth in a sub model comes from either (1) net subscriber growth (players gained - players lost), and (2) increased subscription rates. Competition and a saturated market prevents SE from raising subscriptions without crippling their userbase. We can both agree that is not an option. This leaves only one real option, which is to gain new players significantly faster than you lose them, and to do so at a greater rate every year going forward. That's a tall order, especially when you realize that it costs money to acquire each and every new player.
This creates a problem. The only way to grow players at a faster rate is to throw more money at gaining players. When you factor in competition, new releases, etc etc., your ROI goes south rather quickly. This is normally where games go on life support and maintain a status quo, *if* a company feels it is worth it to do so. It doesn't cost *that* much (relatively speaking) to run an MMO. Its just there's an opportunity cost associated with keeping a game alive, as that money could be spent on a game that will actually generate a lot of growth/revenue. Every company's decision on this is different, but in SE's case they kept FFXI up and running with a few changes. I'm guessing FFXIV had a lot to do with that decision.
Long story short, a strictly sub-model does not meet the growth standards necessary to justify SE throwing a lot of $$ at the game. Whether or not you agree with having those kind of growth standards in the first place is another discussion entirely.