This is not the issue. Your nameservers are pointed to one IP only (164//68//117//127) so if that goes down, all DNS requests to your names will fail until that server is back online.
Plus, using only one server is not ideal because users in one place might have a really high ping to that server. While with multiple servers, the latency is lowered to the closest DNS server.
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I agree with the first part – and with the second part in general.
The algorithms for selecting the best, or fastest, nameservers is different for all implementations, and many variables are at play in the overall resolving process.
For this user, just choosing two different IPs in different netblocks (even better: in two different BGP ASNs) will improve the situation for resilience reasons. For better performance, there are indeed more advanced CDN, GeoIP and GeoDNS solutions available. However, GeoIP/GeoDNS is not perfect. Users will not always be routed to the fastest possible servers. Furthermore, when you implement this, you have to do this for all protocols (also for the webserver) to have the most effect.
The most important question is perhaps: will the user arrange this himself with his own solutions, or will he use platforms that have already arranged this well worldwide, and have a lot of experience in this.