[lucian1900]

Not only is that slow, but it raises errors programs aren’t currently handing. It would be incompatible to change.

[belter]

As opposed to the current default, that also raises errors programmers aren’t currently handing :-)

[lucian1900]

The current default has effect anomalies, but doesn't raise serialization errors.

It's possible to write software that relies on READ COMMITTED and behaves correctly.