It's called Two-sided market place, when the platform enables exchange between two distinct user groups. In fact Netflix, Uber, Airbnb etc. are all Two-sided market place, but I agree that we cannot just sight Netflix as a justification for any subscription.
There could be others who feel commission rates for freelancing services are high and might indeed prefer a flat fee service as an alternative. But, solving the chicken-and-egg problem would be much more critical i.e. having enough employers and freelancers to begin with as the competitors are huge. Some of these platforms, even offer life time zero commission rates between a freelancer and the client they have brought from other platform.
So I feel a change in the business model alone wouldn't cut it as the existing players can always drop their prices, more needs to be done to understand other underlying core problems/need gaps faced by the freelancers and their employers to create a platform which solves them.