Lets assume I'm playing on a server. One day, the server updates and I have to update my pack. I log in, and the first thing that happens is that I fall to my death. I jumped off a cliff before I realise that my Thaumostatic Harness is gone. There's a big empty room in my base where my Thaumcraft stuff used to be. My farms are no longer being harvested, my elevators are gone, my larger rooms are all dark and full of monsters because the lamps have gone. Am I really going to carry on playing on that server? Does that mean that the pack is "riding on Thaumcraft's coat tails"? Thaumcraft is a major mod. One of the top four (IC2, BuildCraft, Thermal Expansion, Thaumcraft, I'd say). Any pack that includes it would be severely crippled by it being removed.
Let me be clear about this.
Azanor wouldn't be "that guy" who simply has a modpack remove TC from it's lineup on a whim. He wouldn't do that without good reason, such as his conditions aren't met, like he's not appropriately credited for his work or the modpack makes money for whatever reason without Azanor's knowledge, so for your hypothetical server to be forced to remove Thaumcraft would most likely be under an understandable set of circumstance, in which case I apologize, but I have no sympathy.
Now, let's assume it was on a whim that Mr. Azanor had your server pull this mod from your lineup. It would indeed be unfortunate and unfair, I agree, but at the same time that's not for you or your server to argue with. Thaumcraft belongs to
him. He programmed it, he has the right to distribute it as he chooses and if someone distributes it without his permission, or consent, then they are infringing upon his ownership rights, regardless if it's fair or not and this is a very common practice that transcends the minecraft community itself, this is basic patent law. It's a dick move, man, I couldn't agree with you more in that respect, that's why I can't see your situation happening without good reason.
What I mean by "riding on the coattails" is a modpack's success or play-ability revolving around a single staple mod. It's by no means a bad thing, but you go into this knowing the risks involved. Take a modpack like Vazkii's Thaumlite for example. It was created with the sole intent to build upon Thaumcraft and mods that compliment it, such as addons and other miscellaneous mods. That uses Thaumcraft as it's foundations but at the risk of losing it's very purpose of being based on the mercy of it's creator. Bare in mind however that Thaumlite was created at a time were permission was needed directly, in which Vazkii had that permission and yet still provided due credit where it was due as to Azanor's request.
My point is, yes. I agree. It would be very unfortunate to lose such a staple mod from a line-up, but that's all it would be. Unfortunate. You would inevitably kick, scream and rant but it certainly wouldn't and shouldn't be the end of the world. The moral of this story is to abide by Azanor's friendly requests and you needn't fear the loss of the mod. It's really not a whole lot to ask when you think about it and when it comes down to it you'd find it just as reasonable if you were in his shoes. I know I would.