DNS itself isn't that complex. We can ask for multiple things (A, AAAA, MX, NS, TXT, ...) and servers form a hierarchy so not everyone has to directly talk to the server that has authority over a domain. Maybe add DNSSEC if you feel fancy. That's about it.

The complexity comes from actually running these things at an enterprise or even global scale. Giving out different replies depending on who asks so everyone can contact a server that's geographically close to them. Load balancing between multiple nameservers. Aggressive caching. Failovers. Rights management. The reasons why DNS is the culprit for so many outages are a) the complexity we have layered on top of a relatively simple protocol and b) a lot of other stuff relies on DNS so problems spread super fast.