I think there are two problems here. Firstly, links that go nowhere - which included a 404 page - are wrong, because they imply that a site is broken. If you have nowhere to go, then don't provide a link to go nowhere. As a rule - there are some occasions where it might be acceptable, but it doesn't speak to me of a good site.
However, there is also a problem with disabled links and buttons, because they imply to me that this is a feature that I do not have access to at the moment. It suggests that if I buy a full license, or log in as an administrator, then I might have access to it. If it is never available then it is merely fluff graphics - sometimes acceptable, but needs to be justified as this.
The issue with removing the link entirely is that there might be design problems with this. However, I think it is better to put some work into making the design work with or without the buttons or links than to have links that are meaningless. It is the same rule as anywhere else, that if the graphics are not doing something useful, then get rid of them. As the usual purpose of links is to link somewhere sensible, then to have them there without this requires other justification.