[WorldMaker]

To my understanding, MSIX today supports the full MSI catalog and will do entirely unsandboxed (unattended [0]) installs if you want it to. But you need to understand all the same complexity of MSI to build installers in it. The biggest remaining difference MSIX and MSI is an MSI is a strange nesting doll of ancient binary database formats that is tough to build (which is why WiX exists and is part of why WiX is so complex) whereas MSIX is "just" an ordinary ZIP bundle of your files plus XML manifests. With the final punchline being that those XML manifests have yet another dialect from WiX's ancient MSI-database-influenced XML and of course it also isn't as simple as deleting the WiX compiler and just zipping a WiX project.

In my experience, you can pretty easily write the nice sandboxed MSIX manifests by hand, it's not too bad, but general MSIX doing weird MSI things you still want better more expensive tools to build them (and of course Microsoft themselves still don't exactly provide that and will point you to plenty of expensive third party installer studios for options, many of which are the exact same ones people have been overpaying for decades).

[0] The one complaint I'm aware of is that you can't do custom installer UI and "attended" installs with user choices. There's one MSIX UI and it is barebones but acceptable. That's all you get.