Re: The reason that the Max series need MCAS
AC wisely noted, "Detecting the failed indication via cross check of other instrumentation (sensors) is more nuanced and complex than it might appear at first glance."
'Connecting the dots' to integrate Avionics can take 10 minutes. Dealing with the failure modes can require the other 99.99% of the engineering budget.
I've seen it done very well; it took over a year.