Then you're not using any of their services besides DNS, at which point you don't need to use Cloudflare at all.

As soon as you turn on any other service they offer, you need to actively migrate away. It's an inherent issue of services that actually provide a benefit. If you're saying "I can just migrate to any other nameserver" then you're telling me you have no use for Cloudflare in the first place. Because if you did, you couldn't just not use it anymore.

Let's say you're using their WAF. Sure, you can just change your domain's nameserver and you've migrated away. But now you no longer have a WAF. Same for their CDN. Or their load balancer. Or their object storage. Or their CAPTCHAs.

I think they also lock you into their DNS when you buy a domain from them, unlike other registrars who allow to change your NS freely. Sure, you can just transfer the domain elsewhere for a small price, but the point is they go the extra mile to force their NS, which I havent seen with other registrars.