Prison time won't bring back the money, meaning it is really just to satisfy a primitive urge for revenge. So what are you talking about, at that rate I won't do it, seems to work, see how it deterred SBF?!?
Almost everyone who commits crimes while expecting to get caught is in desperate circumstances (near starvation, jail preferable to available housing, loved one in big trouble, etc.), where the time isn't a big deterrent, either.
It's much more effective to increase (at least the perception) of the probability crimes will be caught and prosecuted. Excessive jail sentences presumably result in greater costs, meaning less money available on the investigation and prosecution side. (Yes, this makes faulty assumptions about how government budgeting works in practice.)
People are vocalizing their lack of trust in the government to serve justice in part because cases where people whom they feel deserved more punishment didn't receive it and similarly with people whom they feel didn't deserve as much punishment as they got.
An example that comes to mind is that for all the layoffs happening lately in the tech industry, a common sentiment here on HN seems to be that CEOs are talking about taking responsibility but usually don't really face proportional negative consequences to demonstrate how they're doing that. Similarly with criminals, of course you can and should rehabilitate them to prevent re-offending, but if that's all you focus on, you kind of miss out on making them take responsibility for their actions.
Doubling jail time probably doesn't move the needle much, but I would be very surprised if there wasn't a significant behavioral difference between "go to jail" and "don't go to jail at all".