Rust wouldn't be a better language if nightly were instantly made stable. Instead, we'd have a ton of half-baked features that we'd be supporting for eternity.
Rocket requiring nightly is unfortunate, and I wish it didn't, but aside from Rocket stable has been a good forcing function to get crate authors to not add nightly dependencies.
Anyway, from what I see, actix seems to get more buzz than Rocket these days, precisely because the former works on stable.
You're getting pretty far away from the original claim though. Which widely used crates require nightly to take advantage of nll?
Some crates will always require nightly because there will always be some people that want to take advantage of the latest features. That's a good thing to a degree, because it encourages experimentation with said features before stabilizing them. This is completely different from implying that nightly Rust is required by a lot of popular crates, which just isn't true. Most people are productively using stable Rust.
Rocket requiring nightly is unfortunate, and I wish it didn't, but aside from Rocket stable has been a good forcing function to get crate authors to not add nightly dependencies.
Anyway, from what I see, actix seems to get more buzz than Rocket these days, precisely because the former works on stable.