convert it to a runtime that is twice as efficient and reduces your cloud spend tangibly
Smart companies are moving away from the cloud, smarter companies never migrated in the first place. It turns out that scale is a problem that everyone wants, but almost no one actually has. Of those that do, a sizable portion would be better off scaling vertically, the rest can save an absolute fortune by repatriating. I've often pointed people to this article about the architecture of Stack Overflow. That's from 2016. What would you need to do the same with modern cloud-native best practices? Not long ago, the cloud was all about cost-savings in the short-term. Small companies with a fist-full of VMs might still benefit, but the sales pitch now is all about investing in the future. It's very silly.
A LOT of banks still use COBOL and are still doing so because they don't want to deal with the risk or cost of porting it.
The world runs on COBOL. Not just because there's a lot of legacy code out there that's too expensive or risky to replace, but because it's really very good at what it was designed to do. (You mention performance, but you won't do much better than COBOL on big iron.) There's a reason countless multi-million-dollar COBOL to Java conversions failed in the 90's. (In hindsight, we're very lucky that they weren't successful!) There really isn't anything else that comes close, which is why there are still new COBOL applications being developed to this very day. While few and far between, they'll be among the few that won't need to be rewritten in 5 years.
I assume a lot of businesses have useful VB apps lying around...legacy Ruby...PHP...Python/node.js apps that are painfully slow. We'd see a massive boom just from everyone getting all their apps into a modern state.
You're kidding, right? You'd have to be. Well, Ruby and Python are probably a real problem, but let's not pretend that modern software development practices produce anything other than horrifically bloated and slow monstrosities. You're unlikely to find any performance improvements 'modernizing' a typical LoB app, let alone a "massive boon".