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As someone who has looked into deploying a DNS based CDN, I have frequently found ISPs ignoring TTL.

Routers handle bgp updates much faster. Anycast routing is much more superior to geolocation which gives a completely incorrect location as high as 10% of the time. Anycast ips are also the best way to handle DDoS attacks. Attackers can easily shut down ips listed in DNS while people's ISP and browser repeatedly try to access the same dead IP for hours.

If DNS was an acceptable option, I would have gone with that instead of paying a CDN for an anycast solution.

These are all from my personal observations, they are not something I just heard from "ISP/networking people".