Hmm, how does unfinished work differ from unfinished repayment of a debt (for example a mortgage)?

In my view they both require continued ongoing maintenance to prevent negative consequences.

And they both can be intimidating and/or not feasible to “pay off” at the current time.

Just like financial debt, technical debt means accepting an ongoing burden and higher overall cost for a near term gain and avoidance of immediate lump sum payment of money/time.