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
I said engineers/miners making the decision - I should have typed that engineers and miners- those working on the network and involved in the software of the blockchain are the engineers I'm referring to -- not just the miners. Agree with the gist of what your saying but as many any bitcoin buying it as a store of value in addition to its purchasing value I don't think we view ourselves as typical "consumers" where the network is supported by corporation - it is distributed and more democratic than the corporate - consumer relationship. The analogy has some value agreed.
And competition can be good for sellers too but that is a whole different discussion. Competition can cause a seller to introduce a better product that is even more beneficial than the first. Not everything from econ 101 applies in a black & white way in the real world. Supply / demand curves aren't straight lines, competition isn't always bad, consumers do not always seek and pay the lowest price - luxuries are bought, etc.