Even to zeroth order, this is a bad way to estimate backwards compatability. It's a fine way to estimate pace of language evolution, but that wasn't your claim.

For instance, Fortran 2018 has new features that would fail if you tried to use them in Fortran 2015. However, Fortran 2018 is still backwards compatible with Fortran 2015, and indeed is backwards compatible with Fortran 1977.

That is, in this example Fortran maintains 42 years of backwards compatability, yet only 3 years of forwards compatability.

The two things are effectively decoupled from eachother.