This line of thought is bullshit.
When you buy a product you also buy it because:
1. you expect long term support
2. you believe in the company's values and business model
3. you trust that company to keep protecting your interests
There are many more factors going into a buying decision than "current implementation has features X or Y".
Many people are upset with AgileBits because when they bought 1Password, they did so believing that the company's strategy is aligned with them — i.e. at that time 1Password was THE offline password management solution, the other popular alternative being LastPass, a solution that relies on web interfaces and browser plugins and that has had vulnerabilities because of that.
I should also remind you that by buying a software product, you don't actually own it, you're just licensing it — the difference between a one time fee and a subscription being one of contractual obligations on both sides.
But that difference is irrelevant for the topic at hand, because you don't own the 1Password version that you bought, you cannot make improvements to it by yourself, it's not open source after all. When you buy a car for example, non-shitty / classic models at least, at the very least you are free to repair it and improve it by yourself.
And while true that AgileBits has no contractual obligations to keep delivering updates to their standalone version, I personally couldn't give a shit about the scope of their contractual obligations. Because in society when interacting with other people, we also care about other things and we are free to associate or disassociate ourselves from other people at will and the same rules govern our interaction with companies as well.
So when AgileBits no longer delivers a standalone Windows version, that's a serious reason for concern and a flag that we should look for alternatives.
AgileBits is free to do what they want of course, but we can also vote with our wallets after all. And we are also free to complain.