I don't see any reason for Valve to lower their cut for non-AAA games right now - this isn't luring people away from steam. Sure, if the MS store improves, if it actually becomes something with a large and willing userbase, then indie devs can use that as a tool to negotiate a better cut with valve (one of the many tools btw, I'm not at all saying that devs are at all dependent on some competitor store to demand a better cut). But as it stands at the moment, it's just a move by MS to make developing for the MS store more appealing, it's not consumer-focussed at all and that's what ultimately matters. There is no base for negotiation, if there's no actual alternative to the status quo that would hurt valve.