> "When I worked at reddit, it took a month for traffic to shed off of the old load balancer, despite me lowering the TTL to 5 seconds"

Likely because the clients just weren't doing DNS lookups. I would often run into clients written to resolve a hostname and then query the resolved IP for the duration of the process (often this is done as part of a library or runtime). Our customers wouldn't restart their apps for weeks or months so there was always a long tail of clients which never picked up dns changes.

DNS wasn't designed for failovers. We can use it for that, but it's never going to be perfect by design.

Because of this, IP level failovers are the way to go wherever possible. Routing is designed to handle this sort of thing in the best possible way.