Web browsers have a hard constraint of backwards compatibility. Eventually we could see some CSS directive which enables "modern" default styling, both for mobile and desktop, but it would be opt-in so that existing sites which rely on defaults do not break.
I bet if they did an experiment where they changed the default styles a bit very very few would complain. Most people visiting tiktok or whatever wouldn't even notice.
Think for a moment. This has nothing to do with tiktok. This has to do with your favorite personal website which hasn't been updated since 2001.
And any change in default styling would be immediately noticed. Pages suddenly centered, or half the width? Increased padding?
And "most people wouldn't notice" is a harmful mentality in web design which allows minorities to suffer. Most people aren't blind, either. Should we not worry about them when designing websites?