I understand, but that's not the point. TT doesn't support it now while a competitor does. No one is going to hold onto their computer forever, but pissing off a lot of current customers by failing to offer the product for their OS is not likely to get those customers back once they go to the competitor who has a history of having a great product and is less likely to abandon them with requiring new hardware purchases more often. As it's already been shown here, and as I know from personal experience, it's not difficult to use the current Mac development tools to use the same software code for the current OS to create a version for an earlier OS. Many developers do that routinely.
It may be a business decision, but it's clearly pissing off a lot of Mac customers who are going to H&R and unlikely to return even if they upgrade their machines and OS. Intuit didn't support the Mac users very well for Quicken, and they left Quicken in droves for alternative options and never came back. Intuit killed their own market for Quicken Mac. However, the new owner is doing quite well and has a strong following because they actually try to support the Mac customers.
