Because they can’t support out of production products forever. They must support the protocols for the products they produce now and can’t be concerned with a used market that’s trading 10 year old receivers.

Nobody is asking them to "support" anything. Just asking them to allow the product to do what it was originally intended to do and what the manual says it will do. Building new radios that no longer support DSM is the logical way to move away from DSM. If the manual did not say it was backward compatible it would not be an issue. If BMW updated my car to run on Diesel instead of gasoline tomorrow I would not be happy about it. If I wanted a diesel I would have bought a diesel.

I will say the emails with Horizon have been interesting as they have updated their ONLINE manual and taken out the backwards compatible language. Luckily, I have a copy of an original manual that states it. Honestly, their customer service about this has not been great. On the other hand, I had no expectations going into it but did not expect the guy to lie and say it was never backwards compatible and I should just buy a newer radio system.