The Wikipedia namespace isn't editable with the visual editor though (at least not in a way that makes sense - without visualising a wall of unintelligible code, at least). That's why several wikis have a specific sandbox they use for the visual editor, https://en.wikipedia.org/wiki/Draft:Sandbox?veaction=edit&action=edit being the one for en.wp.
I think the easiest way to fix the sandbox issue is properly signalling the "sandbox for the visual editor" in relevant documentation.

The Wikipedia namespace is editable with the visual editor, and it works fairly well (except for huwiki’s header template, but that doesn’t work on mobile, doesn’t work with live preview, doesn’t work in print and so on). And even if several wikis have specific sandbox in another namespace, even more wikis don’t have an additional namespace to place sandbox in (the default namespaces are article, user, file, help and category). The English Wikipedia does have, so they could work this problem around – but it’s a workaround, as the sandbox isn’t a draft, so it should not be in that namespace.

If you want to force visual editing any page, you just need to provide a URL for that. This is the one for en.wp, for example, and I certainly hope nobody inflicts that on any human, Being technically possible (which BTW is what I said) doesn't mean it should be done. There's a reason if the visual editor isn't enabled on that namespace, and users aren't allowed to switch from the toolbar. The location of the sandbox is merely a convention, and changing a convention or documentation is way less a workaround than the hack of forcing the editor in namespaces where it's not enabled. (however, these are just my2c.)

Placing the sandbox in the project namespace is merely a convention, but it’s because that’s the least illogical place. In which namespace should we place the VE sandbox than? Article? It’s not an article. User? Whose? (There are user sandboxes for each user, but we’re talking about a general sandbox for everyone.) File? It’s an error if a file page exists without corresponding file. Help? It’s not a help page. Category? It’s not a category either, and the text “there are no pages in this category” would be even more confusing. If there’s no way showing the VE tab, we won’t write help pages for it. Newbies will remain confused, but I won’t rewrite the help page if I can’t write a first sentence instructing the user how to enter visual editing mode, and keeping the status quo also requires the least effort.

VE is intentionally disabled on discussion pages. Disabling on other project namespace pages seems a side effect. If you really want, make it incompatible with __NEWSECTIONLINK__.