The answer to every security issue not "add a backdoor".

Problem is, I strongly suspect we'd still be having the same discussion even if we were talking about "allow the user direct access to all files*" instead of "allow the user full root rights".

Because while some of those missing capabilities are "simply" a matter of it being too much effort to provide a dedicated capability for each and every niche use case (though that once again raises the question as to whether you prefer failing open, i.e. provide root as an ultimate fallback solution, or fail closed), with file access I guess that this was very much an intentional design decision.