Why do you think so? Once you need to sync with some external source occasionally anyway, why not just run ntp continuously and be actually in sync all the time? (or why would it be better not to do that?)
NTP is insecure, and it fundamentally solves a different problem than the "I need a precise to a second timestamp to validate certificates and update my RTC" need of the majority of devices.
Yes, the base NTP protocol is unauthenticated UDP. So, that's pretty insecure.
Properly configured, with sufficient upstream time servers, etc... it's still pretty robust against DoS attacks and evil maid attacks, so you'll have to do some work to trick clients into following your fake NTP server. And it will be hard to hide what you're doing while you do it.
It took a while, but I think we've actually solved that security problem with NTS. Now we just have to get the vendors and the community to support and deploy NTS widely.
That assumes your clock is running at a reasonable rate, and that the clock speed doesn't vary, etc....
The more your clock is skewed, the more often you need to check to see how badly it's skewed and apply whatever corrections are necessary.
That said, I'm a big fan of running SNTP as a client, where you don't need the full power of the NTP protocol. There's no sense running a full blown NTP client on most systems.
I like the idea of memory safety, but that's just one category of potential security vulnerabilities. If Rust can solve for that and also give me other benefits like higher speed, then I'm all for it. But you have to give me a lot of good reasons, or a hell of a good single reason, if you want me to toss out everything and replace it with Rust. And IMO, memory safety alone is not a good enough reason for most use cases.
LE certs are normally cycled days ahead, so you need only +/-1 day precision for that specific purpose. But the question was: if you need to sync from time to time anyway, why is staying in sync using ntp a thing we wouldn't want?