Hindsight!
'Enterprise' was cancelled last week after only four Seasons. The first Trek TV show not to have run for Seven Season since the original season was cancelled in the sixties. They tried to cancel it last year but it got a reprieve (which they deny was anything to do with getting the 100 episodes necssary for Syndication - something I alluded to in a post above.)
TPTB (powers that be) tried to cancel the original series after two years, gave it a reprieve, then cancelled it after three. But there I think the similarity ends; 'Star Trek: Enterprise' is not 'Star Trek' (TOS), not even in the same league.
It is great that we have this thread here. I just reread it and we identified the problem with 'Enterprise' very early on in the series. TPTB had plenty of time to try to fix it and get it right, yet the retro-trek idea is too much of a minefield for continuity, too restrictive in the aliens and technology it can introduce, and the stories themselves have been too repetitive and recycled.
I'm not sure if TPTB ever gave a denial about guest stars, that would seem unlikely. But they became increasing desperate over time to get ratings at any cost, changing 'Enterprise' to 'Star Trek: Enterprise' (something Rick Berman publically denied), adding more sex and violence (the MACOs, T'Pol & Trip, Orion slave girls), more battles & conflict (the Expanse.)
I'm still waiting to see Season 4, and it sounds like the best Season yet (apart from the alien nazis, the guest stars from the future, the reconning of trek history, and the huge reset button that was anything that happened in Season 3.)
Of course, the 'Enterprise' fans hope that their mail campaign will help save 'Enterprise', in the same way that 'Farscape' fans and 'Firefly' fans, and fans of the original series got some joy (though none of them actually got a new Season.) But as I said, 'Enterprise' is just not the same product as the original series.
And I might add that many better quality series have not got renewed even with huge fan outcries.