Swept back wings and larger intake/bypass engines does not make the airframe inherently unstable.

The fly-by-wire system has multiple redundancies and layered protection, including direct law (at least for Airbus). Fly by wire is, like you say, of course a great innovation for improved control and safety, but it's nice when the plane continues flying even during a failure, however unlikely.

I guess in the case of MCAS the software was activity working against the pilots so maybe it's more of a problem with the design of this particular system and training.