So? I mean, what ist the real downside, beside 5kb additional filesize?
Assuming backwards compatibility.
Ongoing maintenance burden. Ever increasing API, mistakes set in stone, subpar performance or properties.
"Standard library is where libraries go to die." exists as a saying for reason.
So was XML before, and SGML before it. De-facto changes over time, and backwards compatibility means your decisions are cast in stone.
In 20 years, you could see s3 being abandoned for newer formats, but bun will have to keep those packages.