Plus, the only carriers they'd really be able to afford would be T-Mobile or Sprint (or smaller regionals like US Cellular), and their market caps would immediately deplete a big chunk of Microsoft's war chest. This would also immediately make them competitors of AT&T and Verizon, who would then most likely stop selling Windows Phones. This would be foolhardy, since AT&T is Microsoft's best partner. The fact that Microsoft actually owns a chunk of AT&T goes a way towards explaining this.
... on the other hand, insofar as it would be legal, taking a struggling carrier like US Cellular or Sprint and coming up with some sort of "financial aid for pushing Windows Phones hard" agreement might be more compelling. US Cellular gets my attention especially because they're big where I am, and they carry ONE Samsung Windows phone (and only online). They also cover a population/market segment that would eat up something along the lines of the Lumia 735 if it was pushed properly. A few hundred million from Microsoft in marketing support and commitments from the carrier (we WILL encourage people to buy these, we WILL make them competitive, etc) would actually be meaningful for US Cellular, since they're currently operating at a loss.