Good that SAM 4 will be 11 years old this year
Jokes aside, SAM Cloud might indeed be the focus of development in the last 2 years or so, but the SAM Pro edition without the need to connect to SAM Cloud is still available and being maintained as well.
I could understand your critics much better if it was about the subscription model almost every commercial software manufacturer has adopted in the last few years (Office 365, Adobe CreativeCloud etc).
Asking for an old version to be maintained with functionality patches 4 years after EOL is unrealistic, though. A company at the size of Microsoft could maybe afford that, but even they have limited their support periods.
MS provides mainstream support for 5 years after release or 2 years after the release of the successor, whichever comes last - after that MS provides another 5 years of updates for security-related problems only. Applying that same strategy to SAM Broadcaster would make the Mainstream support end in december 2014 and the extended support in 2019, but as this isn't a security-problem it wouldn't fall into the extended support category anyways.
But to even offer such an extensive support you got to have the human resources. First you need the Helpdesk people, that's the easy part as they're around anyways and it probably doesn't matter too much which version you support if you do it for free after you already got the money from the customer for the product. But to actually fix a problem you need to have a developer working on the problem for a few hours. And the developer must first dive back into the old code and look for a solution. Backporting the fix from the current version is usually not an option. That time would be way better spent with fixing problems in subscription stuff (like SAM Cloud) or working on new stuff so you have arguments for selling the next version again.
Spacial is a relatively small company as far as I know. Even though they got a big player in the background since a few years, I doubt that they got significantly more resources out of that. (At least that's the way it was in the company I work for when they were bought by a larger company and not merged into the big company)