If Microsoft buys Activision, wouldn't they then be funding Activision's development? The major difference here appears to be looking at a deal in foresight vs. hindsight.
Sony funds developers = PlayStation gets “extra” games that otherwise wouldn’t exist. No one else is affected.
Microsoft buys Activision = long running Activision franchises (eventually) become MS exclusive. Xbox gain nothing but everyone else lose out on franchises they used to enjoy.