[Lewisham]

Bit of Column A, bit of Column B.

All software tends to unmaintainability, even if you don't touch it (libraries change/deprecate etc). Given enough time, the problems that you are experiencing, say, memory allocation in C, get solved by something else, such as garbage collectors, and the amount of time it takes to solve the headache in what you've got is longer than just rewriting into the new language.