You also don't have to face teams with Sabathia, Burnett, Kazmir, Chamberlain, Wang, Halladay and Shields 18 times each year. The Yankees just added two marquee pitchers and the Rays just went to the World Series. The good feelings from winning two championships since 2004 should be gone; putting that lineup as it is currently constructed out on the field won't be good enough.
If somehow the Sox land Teixeira, the worst they'd lose is Lowell and money. Money is clearly no object, and Teixeira is clearly leagues ahead of Lowell. I don't see why you wouldn't do it, especially since you've already shown your fans you're not going to spend more than you want to.