If S3 was down, R2 would be up but the rest of your app on AWS would probably be down in the affected (based on the cascading failures in prior S3 outages) so most AWS users are going to be down.
If R2 was down and you didn’t rely on anything else that relied on R2, sure, you could repoint DNS and/or issue S3-direct URLs and then clean it up on the way back out.
“Additional options” doesn’t come without “additional complexity” and that complexity has a cost.
R2’s a great cost-saving move here; I don’t think it’s a reliability improvement generally.
If R2 was down could you re-route back to S3?
Diversifying infrastructure provides additional options to mitigate outages.