All very good points, but from my POV you’ve missed the biggest point of all. Experts are already enjoying success and finding enough clients on their own, so they have no incentive to join those marketplaces in the first place.
I see this time and time again. We have a local marketplace of builders and tradespeople, but it’s full of mediocre worker, or worse. All the best builders aren’t on there because their schedules are already busy.
There are countless marketplaces that aim to connect expert freelancers to business in need of their services, but they have the same problem. If it costs any money to be included, or if the marketplace takes a commission on project pay, why would you join when you’re getting a full plate of work without those costs and restrictions? And don’t even get me started on places like Upwork and Fiverr. I’ve been freelancing for 18 years and I’ve never belonged to these kinds of marketplaces.
I don’t know if I’ve ever seen a two-sided marketplace that actually attracts the true professionals in their industry, because experts have little gain from them. They all seem doomed to be full of novices and mediocre talent, because that’s who they offer value to. Which means their usefulness to the client-side of the equation is always limited to those who are satisfied with mediocre work.
No part of that equation is good for experts or for those seeking them.