I don’t understand why you aren’t supporting Win 10. I know one reason is because of security updates. Well, some of us have gotten the extra year of updates. All of the other tax software support Win 10. I have used TurboTax since it was available on computers.
Why? Probably more $$$$ to the bottom line, though Turbotax is not the most profitable product that Turbotax sells. It's also possible that, in the future, desktop will be discontinued permanently.