As I understand it, "tech debt" is something that has to be reckoned with at some point, or else you're going to have real problems in the future (just like refusing to pay off a money debt will generally cause you real problems at some point when the creditor sues you and gets a judgment). You can't just let it go on forever; eventually you need to "pay it down" (by cleaning up the codebase, migrating to newer technologies, etc.), or else catastrophe happens (the company is unable to compete and goes under). One common factor cited in these stories is that the code becomes too unmaintainable and unreliable: too many weird changes for customers pile up and introduce serious bugs which cause the product to not work properly.
This isn't like that at all. We can go on with our current household AC power systems indefinitely. Maybe we could get a 1% improvement by switching to DC systems (at an enormous cost because most of your appliances and devices won't work with it without adapters), I don't really know exactly how much better DC would be (not much really), but what we have now works fine. Furthermore, it's not like the whole electric grid system needs to be changed: it's entirely possible, for instance, to switch distribution systems to DC and leave household systems AC. Instead of distributing the power at 30-something kVAC in your neighborhood and using outdoor transformers to step it down to 240VAC for your house, it could be distributed in DC form, and those transformers replaced by modules which convert the 30-something kVDC to 240VAC. In the old days, this was hard and expensive to do, but with modern power electronics it's not. But even here, the question is: are the gains worth the expense? And the answer is very likely "no". (For reference, I'm not a power engineer, I just studied it in college as a small part of my EE curriculum.)
So this does not, to me, resemble "tech debt" at all. It's just a system that we use for legacy reasons and which is extremely reliable and works well, even though it might not be the absolute most efficient way to solve the problem. This is no different than many other engineered systems. Perhaps you have a decent and extremely reliable car. Could it be better? Sure: you could build the chassis out of carbon fiber, use forged aluminum wheels instead of cast, etc. all to save weight and improve fuel economy. Are you going to do that? Of course not, because the cost is astronomical. There's cars like that now, and they cost $1M+.
So for AC systems that we're talking about, the question is: what is wrong with them that we want to consider replacing them with something else, instead of just sticking with them even if they're not quite as efficient as they could be? Because the cost to upgrade them would be enormous, so you need to have a very good reason.