I'm not sure about Chrome, but Safari had a similar feature but it lacked a way to scope stylesheets to certain domain. If Chrome userstyles behaves the same way then I don't think this change will affect those users anyway since being able to scope userstyles to certain domain seems very important to them.
I don't want to imagine that process getting even more complex than it already is.
Then ask yourself why they are called user stylesheets?
IMHO all these attempts at taking away features "because most of the users won't ever use them" or similar reasons, and then saying "but developers can just do {more complicated thing}, it's better" is a terribly wrong direction that a lot of software seems to be heading in.
"Most of the users won't ever use them" is definitely a valid observation, but how about the ones who do? They probably use them because they discovered that feature and decided to play with it more, learn about it, and found out how useful it could be. By removing them, many more who would've otherwise discovered them will not, they will have a harder time learning about these things that benefit them, and instead remain ignorant. I find this to be almost patronising.
An extension like this: https://chrome.google.com/webstore/detail/stylish/fjnbnpbmke...
As the commit message notes, the current setup already requires putting a .css file in the right location inside the user-data-dir.