"It must be airliner design 101 to have multiple redundancy in all control systems -- but MCAS has no redundancy at all."
It's worse than that.
MCAS uses angle of attack sensors - 2 of them.
Except that it was setup to use a different ONE on each flight - and whilst the flight software could tell that the AOA sensors were giving different readings (even if MCAS was only using one for input), it was an $80,000 option to have this information actually display a warning message (not a dedicated light, just an information message) on the instrumentation LCD - which most airlines didn't bother with as an "option" it's not exactly something that seems essential to a beancounter.
On top of that, if you use 2 sensors on an aircraft, if they disagree all you know is that they disagree, not which one is correct. Critical sensors normally use 3 or another odd number (as with computer clusters, to avoid splitbrains)