If you directly edit the theme file, then on the next update it will automatically be removed.
So if you want to make any changes then you need to use a child theme to do so. If it is only minor css changes, then you can instead use the “Additional CSS” option in the customizer to do so.
Regarding not updating the theme, it is best to to not do so. As with updates, we provide new features, maintain compatibility, fix errors, etc.
Hope this helps.
