Maybe "wasting asset" is a better metaphor. At some point, there'll come an OS release on which that Ruby won't even build (and this can happen sooner than you'd think - 1.8.7 needs to be patched to build on Wheezy, for instance; by the time Jessie goes stable 1.9.3 will probably have been deprecated and unsupported too). At that point, you're forced into porting to a newer Ruby just so you can keep up with security patches to the underlying OS. It might not be "technical debt" per se, but it's definitely a maintenance cost.