Well wait no. If it is the case that anyone can buy off-the-shelf software for self-driving cars for a price of $20,000, then a person could just buy a rig, fit it out, develop a relationship with a set of farmers or whatever, and be in business.

How would one of the conglomerates stop that besides competing on price or erecting regulatory barriers?

Because he's going to buy it for $20,000. The conglomerate can sell it for $15,000. Who's going to buy it from him.

It's a commodity market (so little "customizing" benefit), crazy IP with feedback loop (Google and Tesla have more street data than anyone else, so they can perfect their algorithms. As their algorithms are better than anyone else's, people buy there hardware, which gives them more data, ad infinitum. Breaking into such a market will be getting harder and harder for that reason), and quite likely some kind of (strict but bureaucratic) safety regulation.

er "it", is the software to run your own autonomous vehicles. It is the hypothetical cost of capital. If it is widely available, then people can sell transit services to other businesses.