Braga has publicly stated that the reason behind the Temporal Cols War was UPN wanted something set further in the future than DS9 was, while the creative staff wanted a prequel, so the writers shoved in the TCW into the show to satisfy the execs. I think the shift to arcs happened because the show was tanking in the ratings and the suits were desperate to get ratings, so they let the writers out of the box a little.
Actually, if that's true, then I think the execs made the right call. It should
have been another far-flung future series. The prequel idea was a complete hash. The TCW doesn't fit in with Enterprise as a series at all, but as it's own show it would have had room to breathe a bit.
Studio Execs 1, Brannon Braga zilch.