Question: For example, I want to play all 58 videos uploaded by a single user.

Obviously I could create a playlist, add each video to it, then play the entire playlist, but is there a simpler way?

Answer: 
It's subtle, and it's cheeky, but it can be done:

Given a particular channel URL:

<https://www.youtube.com/channel/UC9mPk8yIk1AUC8PDsUYFBaA>

You can fiddle a little bit and:

<https://www.youtube.com/playlist?list=UU9mPk8yIk1AUC8PDsUYFBaA>

Note that you change `channel/UC` to `playlist?list=UU`.

`$ echo "$URL" | sed 's/channel\/UC/playlist\?list=UU/'`

You don't need to have a particular video.

The playlist can't be sorted, saved or manipulated in any useful way (at time of writing), but when you hit "Play all" or pick a video you can use the shuffle button in the playlist.
If you have subscribed to the user, Youtube TV is an alternative: 
<https://www.youtube.com/tv>

All you need to do is link your account with the app. No apps, plugins, or 3rd party webapps needed.