It extremely doesn't make sense to have a DNS resolver resident in the kernel: DNS resolution is policy-intensive, involves various upgrades to cryptographic transports, and often requires debugging, all weak points of the kernel. Against all that, there's no performance advantage, and doing it admits a new networking API that almost no applications will use --- most new code being written in higher-level languages that already abstract over system calls.
There's a huge performance advantage, if you ask SystemD - Multiple resolutions can be cached. This is incredible bullshit, of course, but that is one of many lies Pottering has gotten away with.
Caching resolutions can be a nice thing, and having in one place can be useful so that the throwing away the cache can be done consistently for all apps when useful (you likely should clear the cache when switching networks, or when the user asks you to), but none of that means it should be a kernel service.
If there's enough volume of requests that a kernel level cache meaningfully improves real world scenarios, something is broken in those scenarios. :p (but I think you agree with me)
