The main reason seems to be clients caching DNS lookups for a period of time longer than what they should have, as indicated by the TTL field in the DNS respons

The main reason seems to be clients caching DNS lookups for a period of time longer than what they should have, as indicated by the TTL field in the DNS response. If this behaviour changes across all browsers, wouldn't that solve this problem? Another "hack" to solve the caching problem would be to have multiple random lookup records (say server-$random.hostname.com) all result in multiple lookups that cannot be cached? The tradeoff here is latency vs availability. As mentioned in the article, triangulation and backup redirection would work as long as "Site A" can be up serving requests labeled (1).