Where does it stop? Who do they listen to? What is "too much"?
That's the problem, and they already talked about those.
By catering to one, you have to cater to all\most, and that can actually disrupt the entire game experience, FOR EVERYONE.
And there in lies the problem, because if the server flops, that same vocal crowd with their custom-made demands will just return\stay on live\legion\bfa and claim "lulz vanilla sucks, we were right all along", whilst the people that actually care for the server, and asked for it for 10 years are left with nothing.
Get it? A game doesn't have to cater to YOUR needs. The game already exists. It was vastly successful, it's popular as hell. Altering it will ruin it. That simple.
I don't really care tbh.
if they don't I'm fairly sure WoW would be over 100 GB. That's a lot of space, and for a lot of players (BfA only, or Classic only) a large portion wouldn't even be something they're using. I'd actually have to swap which SSD my WoW install is on if they don't keep them separate.
It would be irrelevant for BfA players (Vanilla is pretty tiny compared to today's WoW), and it might be a big plus for Blizzard to have people being able to switch easily between current WoW and Classic WoW, so I can see it going this way.
I'd say the real decisive point will probably be how technically feasible it is to replicate Vanilla on the modern client - THAT is probably most of the problem.