What will it do to the other companies selling digital OS maps?
The OS own version will always be up to date. But if you buy digital OS maps from, say, Anquet, you get only the current version - even if it's about to be superseded. (Sometimes companies might allow you to update within a few weeks of purchase but they won't promise to do it.)
The resellers have maps that you download so they can be 100% offline, but at current prices that's not enough to save them. Unless they can agree with the OS to move to a competitive subscription model, they're surely doomed. And why would the OS compete with itself by doing that?
By the way, people writing about motorways and dual carriageways miss the point. There are plenty of road maps, but the OS maps come into their own when you get out of the car.