Most of the time it's a question of trying to apply "death by a thousand cuts" to their codebase, which works well enough as long as you're in the periphery, but eventually they start moving into "core business", you know that entangled mess that has 60 years old code that still runs today, and they realize they need to rewrite all of it, which will take a long time, and cost a lot of money, and they forget about it again for a few years.
It's the same problem everywhere with large and old codebases. You can easily amputate a tentacle here and there, but as soon as you get to the core of it, it is basically one giant monolith, and with age there has been added loads of "integrations" or "shortcuts" between various subsystems, and nobody in the company today has any idea why it is like it is, it just is and it works.
A bank I used to work for had somewhere around 50000 batch programs running nightly. Some were the same program running multiple times, but at least 20000 were "unique" programs. All of those programs had to fit like pearls on a string, each working off of the output of the previous program in the chain.
Untangling that mess is like peeling an onion one layer at a time, with the added bonus that the output of one program might be the final result for some report, and at the same time the input for some other program that needs to do something else.
Add to that, that there's no inherent problem with the mainframe or COBOL. They both work, and reliably as well. Both can push some serious IO through the system, loads that many x86/x64 builds would struggle with.
The conventional answer to IO problems is eventual consistency, which doesn't really work well with finance, at least not if applied broadly. You can get some of the way with slicing / partitioning, but you will still have to deal with a lot of traffic between partitions.
Also Netbeans is my favourite Java editor for hobby coding at home.
The history of IDE market is also about the IDEs that come from OS vendors, and are a much have to target their platforms, at least for those that don't enjoy to yak shaving their favourite tools into the official development workflows from said platforms.
There JetBrains already has scored big time, getting into bed with Google for Android Studio and Kotlin, so much that it wouldn't surprise me if some day Google acquires JetBrains.
Netscape tried to remake Navigator whilst halting development on the old codebase, and it killed them.
Microsoft tried to remake Word, the rewrite failed. Luckily they had continued to develop the old codebase in parallel.
Google tried to remake Gmail multiple times. Every attempt failed.
Apple tried for years to remake MacOS Classic and failed every time. Eventually they had to buy and reskin NeXTStep.
Banks are full of war stories trying to migrate off their old mainframe codebases, and often giving up.
I kinda expected Fleet to die from the day it was first announced. IntelliJ is an extremely mature product that's hard to compete with. They've continually managed big changes to it to keep up with changing fashions and trends in the IDE space, most recently with their new Islands theme that launched yesterday, with integrated coding agents and so on. It's outlasted continuous competition from free IDEs that are always abandoned after enough years pass and whichever executive was championing subsidies moves on or retires (see: NetBeans, Eclipse, VS Express, MonoDevelop...). VS Code isn't so different. Fleet was clearly a reaction to that but the concept was not innovative and focused on reinventing wheels that users wouldn't be able to tell the difference for and which would consume most of their budget, like writing a new UI toolkit, or using a split frontend/backend architecture. Same mistake Mozilla made. Meanwhile IntelliJ was continuously refactored and improved, so Fleet chased a moving target even when they reused a lot of code.
Although people will hate to hear it, the history of the IDE market suggests that eventually MS will get tired of funding VS Code without a big revenue stream to justify its existence. Executives like making new projects and being able to present growth because it represents glittering future potential, but they hate being landed with the maintenance of loss making legacy projects when the originators move on. There's no glory there. For all their problems, JetBrains aren't going to lose interest in their core products due to random executive churn, and that has given their core IDEs a remarkable staying power.