A single or unified marketplace and a convenient paying process can easily capture the whole market, especially if the fees are shown to be artificially small or even zero.

Requiring market places to advertise the actual fee (i.e. anything that doesn't go to the restaurant directly) would probably help here: I might be willing to pay a 5% extra for the convenience, but at 30% I might think twice. It would both improve competitions between marketplaces and marketplace vs direct order.

Of course I'm sure that companies will do anything to game it (for example shifting costs to restaurants to fees not directly connected to the orders themselves).