The CD/download IS the "desktop" software. So---you must mean that you do not like the online software. Sorry. If your OS cannot meet the system requirements for the CD/download (aka "desktop" software) then online is what will work for you if you want to use TurboTax.
I think you mean the online version is insufficient.
this is something that is probably mostly on Apple, and not so much on TurboTax. Apple is very aggressive about removing support for older operating versions from their software development kit (SDK). Once Apple removes older version of the operating system from the SDK, the only way that a software company can support the older version of the OS is to use an outdated SDK. This can create problems with support for the developer, and it also means that the developer can’t take advantage of newer OS features that are only available in the newer SDK.
Windows is much more focused on backwards compatibility, but this also means that windows can’t roll out new features as fast, and it means that newer versions of windows still contain vulnerabilities and problems that have been included with windows for many years.