It's crazy to me that anyone would still consider S3 after R2 was made available, given the egress fees. I regularly see people switching to R2 and saving thousands or hundreds of thousands by switching.
If your compute is on AWS, using R2 (or anything outside of AWS) for object storage means you pay AWS egress for “in-system” operations rather than at the system boundary, which is often much more expensive (plus, you also probably add a bunch of latency compared to staying on AWS infra.) And unless you are exposing your object store directly externally as your interface to the world, you still pay AWS egress at the boundary.
Now, if all you use AWS for is S3, R2 may be, from a cost perspective, a no brainer, but who does that?