But ...
1. People don't share how we often think they do. They can be selective. So the idea that you just need 1,000 and that 1,000 is going to love and share your stuff actively is kind of bogus. Maybe 100 of them actually do it, then the 900 others either sometimes (or not at all) pass on your stuff. So you need way more than 1000.
2.There's rarely such thing as a true fan (except, again, in rare instances.) Customer loyalty today can be gone in a second if you say or do the wrong thing. So there's an assumption you're going to maintain all 1,000 overtime that doesn't hold up. That means the alleged multiplier effect of having the first thousand doesn't (always) exist.
3. You might get 1,000 loyal people, but they don't have a ton of money or resources or whatever to sustain doing the thing you want to do. 1,000 enthusiastic people who show up at your band's free show is great, but when only 25 of them show up to a paid gig, that's a whole other story, and this happens a lot more than you'd think.