Sorry, but I do not understanding your complaint, except that I understand it as a complaint about general industry-wide software compatibility constraints.
Keeping backwards compatibility with underlying systems in software is possible only to a certain extent. Foreseeing forward-compatibility with new systems is (nearly) impossible.
The version of CBSubs that you first bought was compatible with the released CB version, and you got during its maintenance period upgrades to newer versions, at all time supporting newer CB versions that supported newer Joomla versions, that supported newer PHP versions on newer Server operating systems.
That's an industry standard way to manage software quality to avoid exploding exponentially the test cases and be able to add new features taking in account new features of underlying systems.
Anything below Joomla 3.6.5 is not fully safe at this point, and anything below Joomla 3.6.4 is unsafe to use. So there is no point in supporting any earlier release, as it would bite you back.
Similar is true for any complex software.
CB 1.9 is not compatible with Joomla 3.6 and is end of life. Newest version is 2.0.15, and soon 2.1 and free.
CBSubs 2 and 3 are not compatible with Joomla 3.6, and are end of life. Newest is 4.1, GPL, not free, but comes with a full year of upgrades and support.
When you buy software, any software, specially for the web it is of crucial importance to maintain it up to date to keep your site secure. That includes your server OS, webserver software, PHP version, Joomla, CB, CBSubs and any extensions you are running.
Currently your Developer membership is expired, just reactivate it to get another year of new releases and support for CBSubs or get just a CBSubs 1 year subscription if you don't need anything else than CB and CBSubs. There will be new CB versions and CBSubs versions for new Joomla versions all year long. Maintenance is not a one-shot thing, it is an ongoing task and duty to your members. It also includes priority support.