These PE-owned companies create market demand by being shitty. Somehow they are able to keep their margins and their market so that their business model makes sense. That's a puzzle, right?

There is no shortage of investment looking for great returns. A market with huge demand not being met adequately is a dream to investors. Even more when you know the competition must continue to fuck their customers because they paid above market rates for the purchase and the business is saddled with debt obligations it must meet (Leveraged buyouts do that).

What could stop new competition from beating them out?

It's not capital.