> If it were possible for startup telecom companies to compete with the big 3
One reason to regulate markets is to preserve a competitive environment where this is possible. Big, dominant players in a market want less competition, not more, and left on their own they find ways to prevent it.
Look at how Microsoft used the Windows monopoly, for example, to eliminate competition in office suites and web browsers. They made it technically difficult for customers to use other products, they pressured vendors into selling Microsoft's products over others', they used their OS monopoly to give them a technical advantage in applications that ran on it, they distorted open standards to lock in their products ('embrace, extend, extinguish'), etc. It took a government lawsuit to open the web browser market and even that was too late for the competition, who were out of business. (The office suite market never again had competition until maybe the online suites now, and I don't know how much market share they have.)
> competition in telecom would likely enforce net neutrality on its own
I don't see why that particular outcome would be likely. Plenty of industries do things, sans regulation, that are bad for consumers and that consumers don't like. Look at the fraud in the financial industry, for example, or the privacy problems in IT products, or all the nightmares of the health insurance industry.