The above is very readable and means "viewing topic with id 50901", it has a more appealing format for sharing, and it is similar to how YouTube video URLs are:
"Viewing video with an id of dQw4w9WgXcQ"
The canonical URLs are already in the suggested format, however, those are only presented at the users of search engines and not to the users of the forum who see a slightly more complex format that has an "f" variable (it's the id of the topic's forum) in it, like this:
The need for the "f" parameter has been discussed in the past and the conclusion was that it is only needed for the "who is online" list of each forum. Simply put, without it in the URL, a user does not appear in the list...
...and that's a bug because those users who are visiting through the canonical URLs won't appear and so they will be ghost users:
Thus, by dealing with the above bug and removing the "f" parameter we not only fix a bug but we also end up with prettier, more readable URLs.