Backwards compatibility is silly. It is extremely rare than anything would break on an upgrade. In fact I don't recall ever having a single plugin break during an upgrade ever. That includes plugins I've made myself and other plugins I use. The ability for WordPress to upgrade regularly without breaking anything is one of the main reasons I use it.

If plugin developers are having their code break on upgrades, then they're probably doing something wrong (not always, but almost always).

So with that in mind, I'm totally 100% in agreement with Otto and co. We need to find ways to make people upgrade, not provide backwards compatibility which has the exact opposite effect.