The DNS that came with the domain name is a convenience, not an architecture. It will answer until it will not: a high TTL during a migration, no health checks during an origin failure, no Anycast during a flood.
What registrar DNS is good at
Holding a zone while the site is not a product. A few records, high TTLs, mail at Google or Microsoft, no geo, no failover.
What production DNS has to do
Anycast authority, DDoS-protected nameservers, CNAME flattening for the apex, GeoDNS, weights, health checks and a TTL you can actually use. Cloud DNS is that control plane.
You can move DNS without moving the website
Import the zone, preserve MX, delegate NS, then later point the apex at Cloud CDN. They are separate cutovers.
SaaS onboarding is the giveaway
If you provision customer CNAMEs through an API, you are already a DNS product. Registrar panels do not scale to that.