It's not a difficult process to maintain your own repo overlay that keeps old packages for longer than the mainline.
One reason they are trimmed is because old version of $package don't build with new version of $library which is a big dependency (eg vte) that is moving forward so keeping the old ones, and maintaining them gets more and more difficult as time moves on. It's not Gentoo forcing the churn, it's the things Gentoo packages that are churning and it becomes visible at the Gentoo layer.
But your own overlay, or just pinning your build can easily solve that (eg, still using PostgreSQL 9)
One reason they are trimmed is because old version of $package don't build with new version of $library which is a big dependency (eg vte) that is moving forward so keeping the old ones, and maintaining them gets more and more difficult as time moves on. It's not Gentoo forcing the churn, it's the things Gentoo packages that are churning and it becomes visible at the Gentoo layer.
But your own overlay, or just pinning your build can easily solve that (eg, still using PostgreSQL 9)