you can continue using your purchased versions for as long as you like even if you don’t upgrade
This is a valid point only in theory. From a practical point of view the purchased versions will be usable only for a limited time because of the bugs, defects and limitations of your (or any) software.
For instance, today, both Denoise and Sharpen will crash on perfectly compliant TIF files that contain specific tags (or something to that effect). This seems to be a bug on the back burner of the development team and it might still be there in 6 months or more. Most of your customers are not aware of that because the software that they are using upstream of Topaz products aren’t producing the problematic TIF files. If in 2021 a couple popular software start producing these problematic TIF files, it will become a pain for the vast majority of your customers and that will be the point where your point above will completely fall apart. And there certainly are many more bugs, defects and limitation that will pop up.
So, yes you can claim that it is not a subscription if you like, but that rhetorical point is of no practical value for your paying customers. It is in fact misleading and that is a concern whether it is intentional or not. I’ll assume that you are not intentionally trying to be misleading and that you simply haven’t thought through all the implications of your decision and i will strongly encourage you to reconsider.
I case you would consider reevaluating your decision, perhaps you could start from a more practical assessment of the situation and see how you could structure your development and release process in different ways. For instance, you could explore in more detail the whole range of possibilities between “free upgrades” and “paid upgrades”. Maybe there is an opportunity to come up with a long term product plan with major upgrades (e.g. revamping the whole product line with radical new features), minor upgrades (e.g. performance improvement or addition of minor features) and bug fixes. For each level of upgrade you would be able to come up with a rock solid sustaining and retirement plan (e.g. committing on 3 years maintenance for each minor version) which would enable communicating a clear, transparent and reliable plan for commercialization (e.g. buy new for major releases, subscription maintenance for minor releases and free bug fixes for the lifetime of the product line). Some development teams have been experimenting fairly successfully with these concepts for half a century or so, maybe Topaz Labs development team could benefit from these radical ideas. Maybe you would even end up producing better products, reducing the cost of development and maintenance, enabling new market opportunities and avoiding alienating your customers.