> Telling all your users "just get the newer compiler" is rude.
With Rust you don't have the same things holding you back. Features and targets are extremely well supported and for a long time. This compatibility is a die hard sacred tenant. This a good thing! It should be celebrated and shouted from every roof top! It isn't rude that the level of engineering allows this to be a low drama affair. Without the use of RUSTC_BOOTSTRAP it is very rare to have something holding you back on an older, stable compiler. I can think of one major bump of supported OSes a while back and that's it.
You can stay on older versions of the compiler if you really want to, but don't expect to be able to build new code that uses new features.