I think the (unspoken) point of their whole "app" idea is to make people transition into a subscription model instead of a single-payment model; the unification reason was just an excuse.
The only reason is if there is an overriding goal worth the short-term loss of money, and this is the most plausible explanation I could come up with: even if you make people hate your product, if you can force them to move onto a subscription model, they probably won't be able to go back.
They want to unify destops and mobile, so that they can extend their desktop dominance at the mobile market.