And I don't really care about the "help" with invoices and payment etc.
Where is this whole market/business going in the next few years according to you guys?
"Justified" asks the wrong question. The platforms offer a service. If you use the service the platform can profit from the transaction. How long would such platforms stay in business if they had to operate for free, or only get a fee on the first engagement?
Toptal doesn't operate as a platform like Upwork and Fiverr. They hire their people out and make money by marking up the rate/fee.