The comparison would be Amazon just removing non-prime purchases. It’s not possible they’d convert 8 million of those 80, let alone 24mil. The people who aren’t members have already decided Prime isn’t worth buying.
30% isn’t optimistic. It’s impossible. 10% is “optimistic”. They’d be more likely to net a drop in subscriptions when some creators announced that they were going to be forced to stop making content than they would to somehow convert 30% of people who aren’t willing to pay for YouTube.