[AntiImperialist]

That is definitely not technical debt.

Technical debt is the idea that just like you'd take a loan for your business to grow faster, you can take shortcuts while coding your first versions (for eg. not having adequate amount of test coverage, not making it modular or extendible etc.) to get the product out or meet a deadline. Just like your business loan, you get into technical debt knowing that you will eventually pay it back (i.e. write additional tests, refactor etc.).

Not every software problem is technical debt.

While your problem seems much larger, the same problem happens in smaller scales in every software company when engineers quit and leave a complicated codebase behind for new hires to take over. You should be looking specifically for people who have experience working with legacy code in your next hiring round.