The problem is such code can kill a long-running system. Bugs pile up in code no-one understands, or uses anymore. People are afraid to make large-scale refactorisations &#x2F; modernisations to code bases as they will have to figure out how to upgrade&#x2F;fix old stale code.<p>In the case of emacs I can imagine this situation is particularly bad -- lots of old code and limited developer time.