There is no default for unregulated markets. It's a question of whether the economies of scale outweigh the added costs from the complexity that scale requires. It costs close to 100x as much to build 100 houses, run 100 restaurants, or operate 100 trucks as it does to do 1. That's why these industries are not very concentrated. Whereas it costs nowhere close to 100x for a software or financial services company to serve 100x thee customers, so software and finance are very concentrated.
The effect of regulation is typically to increase concentration because the cost of compliance actually tends to scale very well. So businesses that grow face an decreasing regulatory compliance cost as a percent of revenue.
Regulation can increase concentration in a high corruption/cronyism environment — regulatory capture and regulatory moats. There is plenty of that happening.
In building, I think we have local-concentration, due to both regulatory heterogeneity and then local cronyism - Bob has decades of connections to the city and gets permits easily, whereas Bob’s competitor Steve is stuck in a loop of rejection due to a never ending list of pesky reasons.
Inequality at a point in time , and over time , is not nearly as bad if the winners keep rotating
