There are many providers. Cloudflare is on the larger side true, but I doubt any 1 provider holds more than 20% of the total net. Most are regional.
The entire network is connected. That's why you can use any DNS provider and still get to the same webpage. So realistically regardless of which resolver you use, it doesn't really change anything. A user can just switch resolvers on the whim. It really only effects companies who are using services and features reliant upon a specific provider.
The bigger issue boils down to security and bad actors trying to break it. If these actors can ruin AWS, Google, Azure; then cloudflare nor anyone else stands much of a chance either.