It is not true that AMP does not accept contributions. What is your data to back that up?
Here is actual data: https://github.com/ampproject/amphtml/graphs/contributors
AMP currently has 275 contributors (people whose code was merged) on its open source project. Lets aim high and say 50 of them work for Google (probably a bit less), that leaves about 225 other contributors.
Everyone is free to follow, but they don't accept contributions. That is monopolising.