Erik,

Just as an example I can give you the following 'option - no option' suggestion.
We used to use an embedded design with a P89C51RC+. The software and hardware have been developped by a third-party, years ago. Now the RC+ is no longer available and replaced by the RC2 with a x6 clock. Our supplyer can not provide a 100% guarantee that *any* other brand/type processor he is able to supply over a longer term is 100% software compatible. So there's little options left IF one has to supply a number of systems on short notice. So here we go: up to the RC2. Again pressed by time and lack of in-house experience, there is no time to find out how to adapt time-related code in the HEX files we got. Remember: we never got any source code!
Downward compatibility is never requested by end-users; they just want THE part they need. Downward compatibility however IS enforced by manufacturers on their own behalf and comfort just because they stop producing parts. It's as simple as that, I guess.

Regards,

Menno.