What Apple is doing here is really smart. An under-appreciated wrinkle is that grey-market sales are valued on continuous access; you get paid over a period of time, and if the bug you sold dies, you stop getting paid. Apple isn't just bidding against the brokers and IC in lump-sum payments, but also encouraging people to submit bugs early, before they're operationally valuable for bad actors.
Another benefit of this is that researchers are no longer incentivised to hold onto bugs. Currently, you need to find a vulnerability just to get onto the device to do further research. If the researcher reported that bug then they would lose that access. Now they are free to report any bugs they find without jeopardising future research activity.
I guess on average it will reduce those holding on. But it will not eliminate them.