I'm ready to hear your counter-argument. Are you going to tell me Elite has all the necessary in-game tools and those that check third party tools are just idiots?
Well the best prices aren't shown in the 3rd-party tools - or not for very long - so figuring out where they're going to be without them (or at least, without going to the "sort by price" page) is going to work out better if you can. The in-game information isn't great for this but it is usable. But it doesn't really matter.
One of the current top sellers is Musgravite at around 1 million credits/tonne. If there's a 20% bonus for Open, then any of the places selling at >800k/tonne are better than that ... and there's about 25 of them. So you could make more money than the top one in Solo without any significant extra risk by picking the 15th on the list. (And even the top one on the list isn't even in the top 100 traffic systems today, so that might actually be safe too)
With Frontier adjusting the gem pricing so that any of about ten gems are profitable, in different systems, and many miners being happy to take a fair hit to sale price for the convenience of not having to make a 200LY round trip, there's not going to be that concentration ... and so it's just going to be free money for the most part.
CGs, sure. But CGs are about the only thing nowadays that gets people together enough - and no-one does CGs for pure profit in the first place because there's always a
far more efficient way to make money, so a cash bonus to the CG-related payouts isn't really important: you could make CGs pay double if done in Open and they'd still be far less profitable than whatever the actual money meta is. The people doing CGs in Open already are doing it because they like that sort of thing, the people who aren't won't be attracted by slightly more money.
The current best money earner (hint: not mining at all) is so generic that it can basically be done anywhere in inhabited space. Borann and other incentives for thousands of people to be in one system which is also the best earner (and therefore be susceptible to this sort of incentive) was basically a one-off: every other best earner has had multiple options. And when you can earn 200M+ an hour, getting even 20% more only saves five hours on buying a Carrier, or thirty minutes on buying an Anaconda.
(Now, you might feel that even if it doesn't attract a single extra player to Open, the people already there deserve an extra bonus payout ... but then it's not an incentive at all, so why should Frontier bother?)
2. The fact that you choose to carry it in a G5 size 8 prismatic Cutter does under NO circumstance defeat my argument or the premise of this suggestion. It just means you are among the few that have a higher survival change but in no way guaranteed. I'll just pop in with a wingmate with FSD disruptor and you can say bye bye to your load.
With a good parking spot you've got about 30 seconds between carrier and station to find, get behind and interdict. It's not happening very often in that case.
(Why someone who already has a carrier, in this hypothetical situation, is really caring whether they make 6 billion or 7 billion from their hold contents, I don't know - I've already said money is a terribly ineffective incentive for most players)
FSD disruptor has far too much cooldown to prevent high-waking, and a well-shielded Cutter isn't going to lose shields in the first minute of a fight no matter what's shooting at it (cascade torpedoes aside, but it can outrun those) - since it's only going between carrier and station most of the time, it could even have the Fast Boot mod instead of Increased Range.
It just means you are among the few that have a higher survival change
If someone has switched from Solo to Open because of the incentive, then they're probably going to prepare to defend themselves, because if they can't it doesn't work out in their favour monetarily.
So you'd get the same people in Open you always do (some well-defended, some not, obviously you can kill those but the incentive doesn't help because you can kill them today) ... and a few extra people from Solo who would use every defensive technique in the book to be unkillable (which is easy to do with proper preparation).