I really don't see Microsoft blocking unsigned exe. There's just too much old Windows/DOS software out there still in use, sometimes running critical infrastructure.

Like someone already said somewhere, it will come in steps.

Windows S Mode was already a test.

The nagging, warning and outright "blocking" (while hiding the "run anyway" button under "more info") is the first step. This already is a warning to software vendors that something will come.

The next step will be blocking unsigned exes on Home Editions (not on Pro or Enterprise), so that software vendors and most of places depending on unsigned old software can move on to signed software.

Then Home and Pro Editions of windows wont be able to run unsigned software anymore and if you need unsigned software to run you'll have to use an Enterprise Edition.

The last step would be no windows can run unsigned software anymore and if you need unsigned software running, you'll need to run that one on an Azure instance of Windows which can still run unsigned software or (if you can't / don't want to run your software in the cloud) you will have to contact Microsoft for a special Windows version, costing lots of money. But if your business depends on that one single unsigned exe file, you might be ready to pay for that one.