Earlier quoted context omitted.
I think it's still a reduction in risk overall. In the old model, they were vulnerable to S3 failing in one region, a thing that's happened many times. Now they've mitigated the S3-failure-in-one-region issue, at least mostly (though as you point out, how they do so is unknown), and in exchange they've picked up a dependency on Lambda@Edge. But Lambda@Edge, like CloudFront, is a global service distributed across many…
The most famous s3 outage has been operator error from a well-meaning privileged user. The fact that it hasn’t happened for Lambda is just betting on luck. Shit happens, we can’t go designing ever more complicated solutions. May be our services should have some graceful degradation when shit happens instead of trying to create a big-bang and spawn an alternate universe.
I don't advocate for ever-more-complicated solutions as a rule. e.g. I think multi-cloud setups are probably way more trouble than they're worth for most companies.
I certainly agree that graceful degradation where possible and not too expensive is ideal. For example, if S3 is having problems in one region, being able to fall back (gracefully degrade) into read-only mode might be a nice thing to have.
(In this particular case having a secondary region also probably helps with disaster recovery, which is pretty much mandatory in B2B, for better or worse.)