Converting a console's marketplace to local currency should not cause the death of an entire PC gaming marketplace, nor render already-purchased games obsolete.
There has always been a group of PC gamers who loathed Games for Windows Live, preferring other gaming marketplaces, such as Steam. Regardless of one's opinion of Games for Windows Live, there are a number of fantastic games available that happen to have Games for Windows Live integrated into them. Games such as Microsoft Flight and Age of Empires Online will be completely unplayable come July 1st, 2014. Even if you purchased these games through Steam, you will be unable to continue enjoying them
Of course, this is nothing to say of those who DID enjoy Games for Windows Live. The ability to have all of your Xbox Live friends, earn achievements, and, in some cases, play cross-platform with gamers on the Xbox 360 makes Games for Windows Live desirable to many gamers.
Even if Microsoft refuses to release further titles under the Games for Windows Live moniker, even if the PC game marketplace on Xbox.com remains closed down, there is no reason to completely shut off the service, locking out loyal gamers from their Xbox friends, leaderboards, and unearned achievements -- and, of course, forcing fantastic games to close their doors.