More people will be in Open for the prospect of a better reward.
Right, but that's the problem. They won't.
If you're hyperoptimising earnings for maximum credits/hour, then most of those earning methods over the history of the game haven't required being in a particular system, so Open is no riskier. If you're not hyperoptimising, losing 20% for greater reliability is no big deal, not with how fast earning rates are nowadays.
You'd need the ultra-rare combination of the best earning method in the game - by a clear margin - being tied to a specific few systems,
and whatever the second best one is being so far behind that no-one motivated by the reward is going to consider it instead,
and the ultra-rare one not being so good that you can earn more than you'd ever need in a few hours without the bonus anyway. That's never actually happened.
The current best earning method - and most of the previous best ones - have been "new Anaconda every hour" good. At that point, 20% extra really isn't that motivational to someone who just doesn't want to be shot at ... especially since they don't get the 20% extra, they get 100% less, if someone does shoot at them. So even the hyperoptimisers might decide not to bother.
The Epic Games giveaway will have done more for increasing Open population than any tinkering of this sort will achieve. The release of Odyssey (so long as it's not a complete flop, of course) likewise. Player numbers vary by a factor of two, pretty much, just between the weekend and the weekday, and according to Frontier over half of them already play in Open anyway. This sort of thing is going to make a difference to mode played in to a tiny unnoticeable fraction - making the game better in general will bring more players into Open than this sort of thing would, because there'd be more players in general.