When the solution you pick inevitably has downtime too you’re in the same boat.

DDoS mitigation is one of those areas that an on-prem solution just isn’t well suited to solve.

Yeah, but people aren't using Cloudflare just for DDOS Mitigation. Some are running pretty much everything over it, from DNS to edge caching to load balancing and even hosting. That's what I oppose mainly.

Unless you are really big, onprem stuff would be 90% internal anyway. For everything public you'd host your hardware in a datacenter with better high speed connectivity. And pretty much every single datacenter I interacted with in the last 5 years does have a DDOS protection solution that you can order for your network.