I don't know. If they were so bent out of shape that Valve takes a cut of every sale, they could have stopped at any point before now. If anything would force people to use Microsoft's storefront it would have been a new well-reviewed Halo game, but nope, there it is for sale on Steam. And that makes sense to me -- withdrawing from the predominant PC storefront would be a gamble that might not pay off, as anyone who doesn't wish to buy direct from Microsoft is a loss of $60*0.7 = $42 that they could've won buy selling on Steam.

Maybe the calculus changes as they eat up publishers and grow their catalog, but traditionally Microsoft's storefronts haven't done particularly well.