Route 53 General Manager here. Flattening of external provider CNAMEs has a number of availability and accuracy risks. Route 53 offers a 100% availability SLA, and we really mean it. We’ve heard over and over from customers that reliability is our most valuable feature. We can’t provide that same reliability when external queries are in the mix; if we query asynchronously then features such as geo-based routing don’t work as expected for customers. If we query synchronously, then latency and availability are impacted directly.

We do offer ALIAS records between Route 53 hosted zones, and this capability is open to providers such as Netlify. We’d be happy to have customers ALIAS to a hosted zone managed and updated by Netlify. It sounds like your IP addresses are relatively stable, keeping these in sync doesn’t sound like it would be a big deal, and would give you a lever you could pull to change your customer DNS quickly in an event such as this. You could also configure health checks on your own DNS records, which any customer ALIAS records that point to your DNS records in Route 53 would inherit.

If you’re interested in going this route, please contact me at alecpete <at> amazon <dot> com.