Software is an integral part of modern flight. Somewhere up the chain, there’s a highly-paid software engineer/manager who helped design and signed off on the spec.

Would a highly paid software engineer be expected to understand the potential consequences of runaway trim? I’d think that would be up to the aerodynamics folks, who most likely said that it was not a problem.

You are correct, however... I can't imagine a single engineer with any kind of proper education (formal or not) implementing any piece of software or hardware without thinking:"One sensor? Really? What's the failure rate on this thing". That might have been enough to stop the chan of events.

The engineers may not have realized they were relying on one sensor, or they might be the $9/hour variety with no avionics experience who don't know any better.

The main problem was higher up. The overall design was bad, and Boeing didn't seem to have a handle on it.

Or they have a "obey all orders even when it is blatantly criminal" clause.

I suspect they were told that runaway trim wouldn’t be a serious problem, and they believed it since they wouldn’t have the expertise to analyze the whole thing from first principles.

Or may be they pointed it out but got reply "just work as said, you're just $9 engineer"? Or they were never told about the exact purpose?

You get $9 for hypothetical blood splattered on your hand or you get worse than nothing for nothing

If the issue was in the specs, then it is software. Producing code is only really one piece of software development.