I have not yet heard of any workaround to open webloc files in the default browser. I did throw together a web page that will convert a webloc file to an html file that will redirect to the url from the webloc file. The original webloc file will not be modified.
The converter is at
https://www.gradekeeper.com/webloc/
You can convert multiple webloc files at a time. The resulting html files will be downloaded to your computer. Double clicking on an html file will open it in your default browser.
I am still hoping this is a bug and that Apple will fix it eventually. But if Apple really intends to open webloc files by default in Safari instead of the default browser, an html file that redirects to the url might be an option until other web browsers can come up with an alternative to webloc.