It is all speculation until we have all facts.
True, and the "facts" we currently have don't make any sense. No matter what else was happening, any pilot would pull back on the yoke when the nose starts pointing down. This of course leaves a stall as the possible cause for the descent, but how can a 737 flying on a level altitude at over 300kts stall? Next we have to consider an overspeed stall, which seemingly could only have come about as a result of unchallenged nose down flight; so we are back to why wouldn't a pilot pull back on the stick.
It would be strange if only the failure of MCAS caused the crash, as pilots should be able to handle it,
Yes, even panicked pilots don't usually fail to pull back on the stick.
but we must consider the possibility of multiple sensor or system failures and then things might get complicated.
We already have multiple sensors failure: airspeed and AOA.
How about if we make all future airline pilot candidates demonstrate the ability to make a half hour flight in a Cessna 140 without a functioning airspeed indicator or altimeter?