A camera system is far more flexible, can use different magnifications to suit different situations, be more easily angled so you can use the mirrors to see a wider field of view for parking or other situations, be tied up with other sensors to highlight objects like cyclists, that would normally be in your blind spot.
We have had one in at work, the rear vision with the cameras actually has much larger blind spots than a mirror, there is limited adjustment with them and the screen on the door inner takes a long time to adjust to its perspective, also, having used mirrors for 30+ years of driving looking somewhere different takes a while to get used to.