I think Sony would be wise to buy S-E no matter what Xbox does. Indeed, I don't think it really matters what the "competition" does, as long as you have a healthy ecosystem on your own device. And buying S-E helps fill gaps for Sony. It gives them more presence among Japanese gamers who refuse to buy western (still lots of them out there,) it gives them not just one or two but multiple viable JRPG franchises, including (depending on your definition of the term) 3 of the 5 biggest ones. Those same three franchises are also all system sellers and irregardless of Xbox there's danger of losing that to a crossplatform (with Nintendo) release, at least for Dragon Quest. The games also make money on their own, meaning the purchase will eventually pay for itself. And then S-E's anime, manga and JP music stuff can be folded in with Sony's own offerings. The mobile division, despite being distasteful (those FF mobile games are the worst of monetization without gameplay...,) do make money. And according to the finanical results shown here, as much as Square's traditional and MMO games combined. There's also the arcade division which, well, its not really what Sony wants but it can sit in a box for now. Sadly, I think that both Eidos Montreal and Crystal Dynamics fit the mold of what Sony wants in western studios, and Tomb Raider was always strongly associated with Playstation, despite never being exclusive (even the PS1 series was also on PC and Saturn.)
But was it Sony who didn't want those, or S-E? A case can be made for either one not being interested.