Tech Debt, to me, grows with every line of code you write that didn't replace a line of code. Consider how many projects start with the idea, 'I'll rewrite this and get rid of the tech debt'.

First, If the original code isn't deleted, you've added to the tech debt. Now every new engineer will need to learn two chunks of code, and every existing engineer will need to support both.

Second, unless you've had a major epiphany between the old code and the new code, you're more likely to run into the trap of the first example. Major epiphany's could be, 'wow, we really didn't need all of that extra stuff', or, 'we've duplicated so much code, we should re-organize this all'.

I've been at plenty of companies where you have to understand that the Perl code is really old and these components use that, the Mason rewrite was 80% done and is over there, the Next Gen code was in Ruby these components do that. But now, we're embarking on a new Node/React project to replace all of it, finally going to get rid of the tech debt! (Said with no sarcasm)