I should have added another cost of technical debt to your list. Our mental health. If you've ever had the assignment of working on legacy systems that management is loathe to make significant changes to for fear you might break something then you'll understand this aspect of TD. As someone who takes pride in their code and wants to leave the beach a little cleaner than I found it, the "don't fix it unless it's broke" mantra is like nails on a chalkboard. That attitude keeps legacy software creeping toward that event horizon you mentioned. Tech debt can be avoided when you live in an environment of continuous improvement rather than the stasis chamber that most stacks try to live in once the are producing revenue.