Future Guy was there from the start, long before any hint cancellation. Though he was probably added because the suits wanted a time travel element in the show.
But he's from the FUTURE. I'm thinking it was Berman from like 2007 projecting himself into episodes to make it more interesting. In the original timeline it only lasted three seasons so it helped a little.
The reason could have been as simple as, we don't want our universe taken over by the Sphere-Builders. A common enemy for the whole galaxy.
I never understood that--a drive that can take you instantaneously anywhere, I can't imagine Starfleet would ever abandon that completely. Their, um, resolution to that was clumsy at best.