Certain markets, it's absolutely trivial for marketplace participants to agree to an off-market transaction, cutting the marketplace out of any transaction fees. That's what killed all the Uber for Maids businesses of the early 10s.
Uber for Uber, on the other hand, is an absolutely amazing platform for preventing disintermediation. Occasionally, you'll get a driver handing you their personal business card and saying call me directly if you want a ride to the airport but overwhelmingly, people are unable to get around the platform, regardless of how high the middleman cut is.