The part that makes it a non-starter is that it has to be in some kind of a store (whether it's on prem via SharePoint) or the Internet. What is needed is that you can install something on the user's computer (xcopy install, msi, w/e), but without requiring any other Microsoft end-point. And then, the bits installed either run completely on-node (similar to COM/VSTO) or it would require that access to remote web-server be required. If a remote web-server is required (which is a bit of a limitation), it should be something that can be pointed to without Microsoft's involvement. Just like I can create a web-site, give you a URL to it, and you can go it using your WebBrowser. No app-stores.

That doesn't mean app-store shouldn't be an option. I would love to also publish my add-in in the Microsoft AppStore, but it should be my choice, not a hard-requirement.

That's why people are still using VSTO, not because we have a problem with JavaScript vs. C# or COM.

The app store / Office admin stuff. When I was dealing with this stuff 2 years ago, it was insanely difficult to deploy a web add-in internally (something involving the "Microsoft 365 admin center"), let alone deploy it to customers (gotta get approval to deploy it in the special Office app store, and hope+pray that your customers' IT departments don't block that).

Compare to VSTO, where IIRC installation just involves editing a registry value to let Office know where your add-in can be found on disk.