I have a heck of a lot of colleagues at WS and to a person, the ones I've spoken with casually, no one is thrilled about seeing company routes being flown another company. Of course they are trying to be positive about how it diminishes the risk to the company, but to say that anyone is thrilled about it seems pretty off the mark.
WS management are smart but they aren't saints. And this "****ty industry" is only going to get ****tier with oil headed on the rise again, bigtime. Anything that can reduce costs WILL be done, and there's a whole bunch of 737 drivers that just might be surprised to find that a "temporary, seasonal, experimental" etc etc move somehow never quite goes away and actually gets expanded.
This isn't fear-mongering, just basic history and economics.
As an aside I can't really say I would consider the 757 a big airplane, it's a slightly bigger narrowbody than the 737 but that's about it. I would think the operating costs would be disproportionately higher than a NG, what with less efficient engines. I do think it (the '57) is the sexiest airliner ever made, for sure.