My gut feeling is that one can minimally maintain systems in "forgotten" languages indefinitely if they're basically stable and feature complete. But it seems that people come unstuck for systems that still need to evolve -- even if that evolution is slow and/or sporadic. The obvious alternative is to rewrite systems in new languages, but that is generally prohibitively expensive. Even for those that can afford the expense, the new system tends to not to take considerable time to bed in, so flipping the switch from the old to the new is only for the brave or those with excellent PR departments.

Although we didn't have this in mind when we started on this work, I've come to think that language composition might offer a way out of this mess (I immodestly offer [1] as an early example). If one composes an old and a new language, one can migrate a system from an old to a new language bit-by-bit. As well as amortising the translation cost, it also means that one can minimise the "switch flipping" problem: disasters seem much less likely if the composed system can be introduced gradually.

[1] http://tratt.net/laurie/blog/entries/fine_grained_language_c...