With platforms like these the choice isn't really up to the supplier, but to the consumer. The consumers come to expect those value-adds and so the suppliers have to switch to the platform (where the revenue share is opaque to the consumer) to get gigs.
The 20% cut also does sound rather extreme, but if you build a very good product where you can end up charging even more than the usual price, the 20% isn't taken away completely from the supplier, and might end up more like a 10% actual cut.
With platforms like these the choice isn't really up to the supplier, but to the consumer.
Once the platform has a significant number of suppliers, sure. If the platform is new then attracting suppliers is the number one problem to solve, and that's really hard if it's too expensive.
