While it's good to be suspicious, I don't think we can call CloudFlare a monopoly quite yet.
Akamai is a big, giant competitor. You also have the big cloud providers like AWS that have their own CDN systems, like CloudFront. (I don't recall GCP's or Azure's product names.) Then you have specialized CDNs like Google's AMP system.
Now, is it possible that there could be a horizontal trust between these companies? Certainly. There's few enough players for that to happen, but so far, I haven't seen signs of it happening.