At least with regard the with/without slash redirects there is value in these: sometimes the user copy-pastes the URL but forgets the final slash, sometimes the application he is using for bookmarking, sharing, etc. drops the slash; sometimes even the site owner forgets be consistent. Thus having those redirects in place means not having dead links pointing to your site.
With regard the `.html`, with and without, it's a matter of taste... However, the same point applies, perhaps you've been inconsistent along the years, thus having redirects saves the dead links...
However you are right about the "canonical" URL: for search engine purposes, having a single page serving the content, and the rest being redirects, is kind of essential. (Or at least having a `<link rel="canonical" ...>` if not a redirect.)
> Supporting more things for the sake of it is not a virtue. There is value in failing early on incorrect input. Postel was wrong.
It is when you don't want dead links. :)