> I've never seen a company that was a., successful and b., prioritizing tech debt/upkeep at the same time.

Maybe you have only worked in start ups? I have certainly seen products fail or partially fail because of tech debt. One product was extremely unproductive to work on because of a patched together architecture. To the extent that bugs were introduced easily but took so long to turn around that customers left the product and told us that was the reason.

What's the transition threshold at which paying down tech debt becomes worthwhile? Maybe it's a gradient.