The primary sales of FF have always been in Japan, where there is zip XB360 installed base. I don't think there's much precedence for believing XB360 owners' taste care much for JRPG either. The XBox platform is strong in racers and shooters and western games, which suggests the owners want that sort of game. If they wanted JRPGs, wouldn't they have bought a PS2 instead? I guess sales for FFXI on XB360 would give an indicator of how well the FF franchise might do. But it's not enough to say 'there's more of this console, hence you'll sell more'. The consideration is which platform has the more potential buyers for your game, which isn't always the platform with the most units (until you get way out ahead). On PS3 Squenix are pretty much guarenteed about as many million unit sales in Japan as there are PS3s sold there by the game's launch (assuming not a prolonged release). Maybe 2-3 million? Overall they'd need a high percentage of XB360 owners to buy into it to equal sales, with no PS like certainty that those owners want it.
I also don't beleive if a new FF was launched on PS2, it would go overlooked because the 'platform is dying'. The reason to move on isn't a numbers game or money game, but because they want to create better products! They want to try new stuff, and make more immersive experiences. You even get that explanation from the interview. FFXIII was being developed for PS2, and after producing the demo for PS3, they didn't want to go back. If they hadn't have produced that demo, FFXIII would have still been released on PS2 and sold millions (unless released really really late), costing a lot less for Squenix to produce. There's no way they'll make as much money from FFXIII on PS3 as they would PS2.