Dual booting (triple in my case - 14/15/26) is not a viable option for Xcode development and testing of an app because different versions of macOS support different versions of Xcode. While Xcode 26.0.1 currently runs on macOS 26 and macOS 15.6, it certainly can't be run on macOS 14 and I can easily see a time in the near future where some version of Xcode 26.x requires macOS 26.
Dual/triple booting does sound like a way to at least be able to run Test Flight so I can at least do my own verification of functionality before pushing the test build to beta testers.
In my specific case, my app is not a game. It's a productivity app. My primary testing concern is ensuring the UI works correctly under different versions of macOS (and iOS for that matter).
Being able to do this with VMs would make things SO much easier. One copy of Xcode on the host. One copy of the source code on the host. One build on the host. Then run that one binary (stored on the host) in the different VMs. It would be just like testing iOS apps in different iOS simulators. That's all I want from macOS VMs. Just enough to verify the UI looks and works correctly, just like I do with the iOS version. When I'm satisfied, then I send it off to the beta testers.
If macOS 26/Xcode 26 could solve the provisioning issue such that a built app could be run in VMs running macOS 14 and 15 then so many developers would be far more productive.
I understand that testing in a VM has limits, just as there are limits in testing an iOS app in iOS simulators. But so much can be tested within those limits. We just need to have the ability do so.