What's the difference if it was controlled with FBW or by MCAS changing the trim? MCAS, in its designed role, accomplishes essentially the same thing that a FBW system would.
In FBW systems there are no mechanical fallbacks (like the infamous trim wheel in 737). Therefore, any FBW aircraft has to fulfill much more stringent criteria regarding to sensors, computers and actuators. Apparently in FBW it is standard that sensors and even computers are in triplets, so not only a failure of single sensor can be detected, it can also be made inconsequential by voting (e.g. the system compares all 3 sensors and disregards the one that is furthest away from the other two).
What Boeing did is a very bad mashup of old and new technology. They introduced MCAS, but in theory it is not the FBW aircraft so MCAS is not a necessity unless in a very specific circumstances. In theory, the cockpit could have one big red button "all computers meddling with controls off" and would be still flyable. That encouraged thinking "but pilots can disable MCAS", which indeed it is true. BUT due to certification requirements and desire for common type certification prevented exactly this - there is no "MCAS OFF" button. There are only electric trim disable switch(es), which makes situation more complex (it disables both MCAS influence AND electric trim). But even that, in the eyes of Boeing engineers, was not an issue, because there is always a trim wheel to fallback to...
Except, it turned out that a) trim wheel is almost never used b) has been made smaller with increasing computerization of the cockpit (and because it is never used) which resulted in c) the mechanical fallback was suddenly extremely difficult way to recover from the MCAS malfunction.
