You just hack the machines that are loading the data onto the physical tapes or the system that is collecting the data to put onto the tapes. Essentially, at some point the data goes from where it is being used to the tapes and you just takeover one of the systems in that pathway. You then wait for 6 months silently encrypting the data before you make your demands. Now the absolute worst case is that 6 months of data is lost or however long you were hiding. Industry studies indicate that the average time between infiltration and detection of an agent actively exfiltrating data is a few months, so a few months for an agent not even pushing data out over the network, just silently corrupting data going to your off-site backups that you are not looking at is very reasonable.
Backups are not the end of the story unless you are dealing with attackers with only $10k to their name which is essentially what everybody without backups is losing their minds over and being defeated by. That is a literal rounding error of a rounding error of a rounding error for the financial industry. People spend more on lunch than that. A moderately sophisticated attack with a few million behind it is literally 100x the resources of most of these attacks and that is still just a microscopic pittance compared to the financial industry. Think about that, if you want to reach the $1M level you need a system that can defend against an adversary with 100x the resources of a basic ransomware attack. The gap is so large that the capabilities fundamentally change and intuition for how to defeat a $10k attack does not generalize.
And, we have not even considered a system that would even be considered barely adequate for the financial industry. If you want to get to something barely adequate for the financial industry, like say protecting against an attack funded to a level comparable to one day of disrupted operations for JP Morgan, you would need to protect against an attack on the order of $500M, literally 500x more than those "good" systems and 50,000x better than these basic systems. The gaps are ludicrous and the lessons at one scale do not really apply when you go up another 2 or 4 orders of magnitude.