I guess the point of a three year window is to be able as an ecosystem to at some point adopt new language features.

When you have some kind of ecosystem rule for that, you can make these upgrade decisions with a lot more confidence.

For example in my project I have a dependency on zstandard. In 3.14 zstandard was added to the standard library. With this ecosystem wide 3 year support cycle I can in good confidence drop the dependency in three years and use the standard lib from then on.

I feel like it just prevents the ecosystem from going stale because some important core library is still supporting a really old version, thus preventing other smaller libraries from using new language features as well, to not exclude a large user base still on an old version.