(In a previous life I worked at Bloomberg)<p>I don&#x27;t remember the exact figures but 25m lines of Fortran seems plausible, but it was still the minority of the code base. The majority at the time was in C and C++, with probably a few million lines of Perl and Javascript as well.<p>Plenty of the Fortran dates from the 80s, rewriting it in a modern language would be a huge project which lots of risks and limited upside. No significant new fortran has been written for a long long time (&gt;10 years).<p>It&#x27;s also a mistake to think of Bloomberg as a single piece of software, it&#x27;s closer to an app store, there&#x27;s are thousands of specialist apps (functions in Bloomberg terminology) and most users only use half-a-dozen, but which half-a-dozen varies significantly between users.<p>This makes it very hard to displace wholesale. There are individual pieces a competitor can go after but identifying a subset of functionality that&#x27;s enough to convince users to switch is hard.<p>There&#x27;s also a lot of stickiness which goes beyond pure functionality. Network effect and brand are also key parts (having a Bloomberg Terminal is a status symbol).