This is where integrity and vision come into play. If you want to bump it into 6%, you should explain why. It should not be all about profits. If it is, then such platform won't be different from Upwork.
For example, if someone else feels the potential is underutilized, they could offer to buy it and then bump up rates.
How do you structurally make sure that the motive is different?
Some kind of collective idea might work. The model should be the stock market. Trading costs are getting lower, transparency is going up etc.
If bumping up rates would benefit the community, why not?