Murph...
Have to say I am very puzzled that the cells are not individually monitored in the current design since that would give the best early warning of problems.
There are at least two reasons Boeing is not puzzled. (in my opinion).
They were overconfident in the single Temp system, or they knew multiplying the sensors would result in a higher number of Battery failures.
I lean toward the latter; Boeing knew the more data they had regarding the system performance, the more failures they would incur.
This is not sinister, necessarily, it is a judgment call within the rather loose confines of their self inflicted Regulatory restrictions.....
Their decisions were based on on over reliance on optimism....
No harm, no foul.....But Hell to pay if it goes up in smoke....