We left the servers running unmolested and un-upgraded behind a firewall (thankfully they were super stable) until we could replace the 3 unmaintainable microse

We left the servers running unmolested and un-upgraded behind a firewall (thankfully they were super stable) until we could replace the 3 unmaintainable microservices that did something simple in the most complex manner imaginable with a 50-line module within our monolith. (In this case, technically one person on the team - our most sophisticated developer - was able to decipher it. But every time he needed to touch it it would take him 2 full days just to understand the code again. Just wasn’t worth keeping when it was so pointless to have and so easy to replace.) For a more complex codebase, the equation might have been different.