I'm incredibly pessimistic about this. As a general rule, I don't care about prequels, but this is one specifically makes me ask "what story could they possibly tell?". Langford never figured out the Stargate, never stepped through it until she was already grey. The clearest route they have to a story is to come up with something utterly implausible (that Langford WOULD have mentioned in SG-1), and then oh no her memory is erased, or she has to promise not to tell anyone. *gag*
Even if this wasn't its own subscription service, I can't see myself putting in the time to watch it. And as much as I love the Stargate shows, I
seriously doubt many people will find it worth the money just to subscribe to a single franchise. Even if the reviews are stellar (I'd wager they won't be), this could totally fail based off the subscription model, and MGM will assume no one wants Stargate and shelve the franchise for another 7+ years. On the off chance it is successful, they'll take that to mean people want more prequels. Which personally, I don't. No prequels or reboots, I want a continuation.
So from my perspective, they're making something I don't want, with no sign in sight that I'm about to get a show I would want.