davheili , I don't think that's so easy to deprecate old versions and push people to move on the cloud. Don't know concrete figures and where to find them, checked latest annual report. In Productivity (Office products) consumer products give about 15% of commercial products income. In year 2016 Excel had 750 millions users worldwide, perhaps close to billion now. We may assume major part of them are commercial users, and I believe significant part of enterprises prefer to stay on old stable versions. Most probably we speak about tens millions of seats, if not hundreds millions. Same as Military pays (or paid, not sure about current situation) for not to upgrade from Windows XP, enterprises and public organizations are ready to pay some extra for the support not to upgrade from old versions of Office.
Again, I didn't find the stats and not sure if it is published somewhere, just my filling of the situation.