Using bitcoin as an example, letting the miners decide on scaling is akin to going to all the shoe stores in your city and letting them vote on if a new business license should be issued to a new shoe store wanting to start up. It's inherently against their best interest (sharing market cap with a new competitor) to allow more competition, so they will default to voting no.

Competition is good for customers, not sellers.

At the same time tho, your point is valid in that it shouldn't necessarily be decided just by the consumers because most would most likely not be up to speed on the nuts and bolts of the operation and their votes could in turn push the system to unprofitability. To the point of collapsing the system since it would ultimately cost businesses money to continue to provide the service, and we know that businesses exist to make money, not give it away