This is a furphy. GitHub is already monetized -- it is highly profitable, selling fee-paying plans to corporations. MS's major customer is corporations. Of course they're not going to do a SourceForge because this is a traditional acquisition where the purchaser has recognised that GitHub's current monetization ties in well with their existing business model, and they gain value in the acquisition from it building their reputation with business as the go-to place for corporate cloud. They have no interest in damaging the corporate value they gain by fiddling with the consumer and open source side.