People often learn the lessons in a hard way: they will keep saving 230k/yr until one day their non-HA bare-metal is down and major customers retreat.
Sounds like they already have their bases covered.
HA architectures exist for a reason because that last step is a massive headache.
Because TTLs are a guide not mandatory. And many companies/ISPs ignore it for cost reasons.
Not all DNS servers properly observe caching timeouts, so some customers may experience longer delays before they see it working again.