Totally agree. That is why I went on to suggest a slider so the user could set a % suitable for their screen / use case.
Your modified userstyle.css is great for those who have lots of long-form, article-like notes and if you are constraining the width of the text, it makes sense to constrain any other elements (tables, images, code blocks) to match.
After testing your css I saw that it could really help in legibility for long-form text notes or even stored articles. I was just a bit unsure that it would be a good idea to make it the default display style without some very easy way for a novice user to adjust it.
I am not a coder so I have no idea if such a thing is possible. But I use Joplin many times a day and it’s making this the “default” layout that triggers my “spidey-sense”.
Well that’s my tuppence*-worth anyway.
EDIT / Clarification:
* Archaic British term for “two pence”. Please feel free to insert your local currency equivalent here!