For them, official support from MS is not a major factor in their decisions.
Whether it should be or not is another matter, but in talking with them, availability of patches really doesn't move the needle as much as you'd expect.
Maintaining a UWP head and WPF head with .NET Standard libraries shared between them is a reasonable option in some circumstances.
But better still, maybe encourage your employers not to death race upgrades versus exploits all the way to the bitter end in April 2020 like they did with XP's extended support period.
Ikr, but we can't expect all our customer to upgrade just because our product is in UMP. When we looked at one of our products user base, we could see that over 50% used Windows 7. That ruled out UMP for the subsequent project.
An operating system will run as good as it did 10 years ago (just not as good as those released 10 years later)