Well, if you're working on a codebase for 10 years, then "no budget" is not really an excuse, sorry. As a responsible engineer, you should have either convinced management to spend some of your time on refactoring main parts, or cleaned it up yourself bit-by-bit every time you touch something. 10 man-years should be enough for a program that was created in 10 years by a single rookie dev.
Sure if this was my job, then I would do it but I am just contractor with set amount of hours devoted to the project. First few years was spend fighting fires as the company needed this very specific software to function, currently it is just about keeping eyes on having it run and occasional fix some report or update data pipelines.
I would say that rewrite would cost about 2 millions of euros. Which is really big price tag for company that use this system as a backoffice tool.
Some company have back offices that they've spent considerably more on. Airline companies for instance may have a tool that lets the person at the gate check who that person is, what their deal is, etc. And then GDPR happened and the bill to ensure that every rule is followed to the letter and suddenly 2M€ isn't that bad after all...
