Removing Blackreach from the game is a bad idea. I can't imagine any good reason to do it.
First, let's touch on Vengeance. The Vengeance test-turned-feature has been a thinly-veiled deception from the get-go. "It's a performance test, guys! Actually, we are going to add some more features... for performance testing, don't worry. Just a test. Only temporary. Here are some icons for perks! Hmmmmmm, actually, it's not really about performance anymore; we really want to make a game mode to ease more casual players into PvP. Vengeance affords equal footing for people to hop in and enjoy large-scale battles without knowing anything about PvP. Isn't that great?! Vengeance is ready for prime-time!!!"
Stop it. You never fooled any of us. Vengeance has always been the best ZoS could do to treat the symptom (Cyro performance is bad) rather than the disease (properly investing in hardware and software improvements to deliver the experience players expect in Cyrodiil). Since it's been "popular" with casual players (i.e. you put golden pursuits and bonus rewards in there, shut down other campaigns, and then were delighted to find players showed up!

wow!), it's now positioned as the future of PvP in ESO. That's a terrible mistake, not because Vengeance can't occupy a valuable place in ESO's PvP ecosystem, but because it's being forced on an existing community of PvPers who DO NOT WANT IT.
And yes, I mean "forced." Gray Host is not a functional alternative; it's alliance-locked, meaning that any Vengeance-haters who play multiple alliances are out of luck. Who overlooked this, or decided it's OK?!
Listen, we get it: you've invested in Vengeance to the point that you can't throw it out. Casual players like it; that's good for the game. Fine. Let them have Vengeance. In fact, let them CHOOSE Vengeance. I personally doubt it will retain non-PvPers' attention for long and will become a ghost campaign in the absence of special incentives to play there, but that's beside the point of this post.
The point of this post is: leave Blackreach alone. The many players who enjoy ESO PvP, play characters spanning multiple alliances, swap alliances to help friends, and prefer to avoid the long queues and undesirables who plague Gray Host will thank you. And if Vengeance is truly worthy of existing, it will be plenty populated regardless of other campaigns continuing to exist.