In particular, note that new Rust compiles code with both old and new editions. Upgrading Rust does not require you to move to the new edition. And one project can pull in crates from multiple editions.
(Imagine if Python 3 let you import Python 2 modules seamlessly.)
No, there's a commitment to not cut old editions. A new edition/frontend comes out every 3 years, so Rust (and maybe humanity) is probably going to be completely dead long before it has hundreds of versions.
To elaborate on what sunshowers said, because editions only impact the early parts of the compiler, most of the code isn’t edition specific, which makes maintenance easy.
(Imagine if Python 3 let you import Python 2 modules seamlessly.)