Changing well-tested code is unsafe.
we can go in circles all day with blanket statements that are all true. but we have ample evidence that even if we think some real-world C code is safe, it is often not because humans are extremely bad at writing safe C.
sometimes it's worth preventing that more strongly, sometimes it's not, evidently they think that software that a truly gigantic amount of humans and machines use is an area where it's worth the cost.
And if it's well-tested, maybe that condition is achieved by the use of a test suite which could verify the changes are safe too?
Just like SQLite has an explicit compatibility guarantee through 2050. You literally do not have to update if you do not want to.
It's pretty different from the git version, which receives new releases all the time for things like security patches, improvements, and new features.