The more popular it becomes, the more incentive there is for other companies to enter the space and grab a chunk of the money. The barriers to entry are not insurmountable for e.g. existing CDN companies to start offering DNS, WAF and DDOS mitigation products, making them more like Cloudflare. They already have the peering/hosting relationships and engineering teams experienced at working with large volumes of traffic. I wouldn't be too worried about Cloudflare taking over the internet this early on.