The main argument is economical; desktop computers are stale (well, thanks AMD for stirring it up recently a bit), most people now treat computers as home appliances that need replacement once in 5-10 years. In mobile, market still grows (even if showing signs of saturation), there is still a year-to-year progress making newer models appealing and there are more mobile devices that computers with faster replacement rate. So what MS was getting on new end user licenses is lost; they lost mobile as well; the only move left are subscriptions which individual users hate and businesses love.