Yes, as a matter of fact, my first job was about migrating a Delphi + Oracle ERP system where all the business logic was coded as Stored Procedures to PostgreSQL. Things like 3000 line SQL queries and 10,000 like PL/SQL triggers and functions where common. It took us a one year, but it worked and saved the company millions of dollars in licenses alone.
Discussion
So, the license had run out? They often run a decade...
Yep, they had a system for 2 decades and decided it was not worth paying. The 3 of us did the full conversion in a year. And I was a junior, so.. super cheap.
The long contracts is one major reason why nobody bothers. I've read that they sometimes offer a discount, if you agree to not use anything else for the life of the contract, too. Otherwise, you pay the difference and a penalty.
We're about to enter the second year of planning our next big Oracle migration. LOL Every time it gets close, they get cold feet and delay again.
The criticality of the system is off the charts, so any error would end up front page news. I get it. But, at some point, you have to just bite the bullet.
I don't know how big your system is but my experience is that this is a lot easier than what it seems, especially if you have good testing. I was able to build tests and translate 40 plsql stored procedures every day. After you learn the gotchas, the rest is easy. And I can only imagine that the language now is a lot similar than what it was back in the day.