Great idea in concept but the RF noise floor for wifi signals is already terrible. Running a global mesh network isn't going to help matters. Ad hoc and mesh type networks are inherently inefficient and require transmitting the data many more times than an engineered wireless network with centralized nodes.

Decentralized at the local level might work but the telecom industry owns the back bone of the internet currently so there's not anyway around them... yet.

Look up Wireless Internet Service Providers. I think you'll be surprised how many companies are already positioned to break the monopoly.