Oh for godness sake, my first "soft top" was a 1974 Triumph Spitfire back in 1984, that was pre everything safe, no abs, to traction control, no power steering, no crumple zones, no interia reel seat belts, no rollover protection at all, i even remember a friend having a "frogeye sprite" with drum brakes all round. None of my friends who had these little pocket rockets ever rolled one, and believe me in 1984 we drove like hooligans because speed camera's were none existant and speed radar was in it's infancy on roads that definitely weren't as good as today. Not only that this was before you had to notify insurance companies of the modifications made, and boy did did we modify them.
916 spiders have a low center of gravity, so i was trying to imply a number of the factors in rolling one IMHO will include speed(which is the number one cause of most serious accidents)and hitting something solid to pre-empt the turning moments that would induce the roll moments. And being honest if people are soooo paranoid about rolling their spiders , i would say they need to assess are they driving the right car or perhaps are they employing a safe driving style. And of course there is the uncertainty of other drivers whom no-one can predict what they are going to do. You could of course go down the avenue of add steel tubing which will change the rigidity of the vehicle and possibly change the handling characteristics, but where is the enjoyment of summer driving encased in a steel cage, certainly not for me, again my opinion.
However also if you look at the current crop of "soft tops" a higher percentage of theses vehicles do not have substancial rollover protection evident such as visible rollover bars, i did not say every cabriolet had no protection, in the last 15 yrs EURONCAP has ensured higher levels of safety in all vehicles and legislation to encourage manufacturers to make their products safer, however the 916's were intoduced in 1995 and they possibly predated most of the current legislation. In fact the only Alfa Romeo models currently listed in their test figures are the 147, 159 and MiTo.
Please note these are my opinions and mine alone....I'm a firm believer all accidents have causes and most are preventable, alas my days of Police Collision Investigation ended in 2004 after 9yrs of challenging work and I'm not involved in this side any more
I agree with most of your sentiments, low centre of gravity being the major factor for me. My concerns are as much to do with a combination of factors (age related) perhaps coming together, where the 916 Spider seems to be very susceptible to significant issues which ultimately affect handling ( wishbones, bushes and bearings etc) I bought mine to enjoy open top spirited driving (especially through the twisties) which is where any mistake or problem is more likely to occur resilting in a flip. I'm also 6'3" tall and sit quite high in them so perhaps feel a little more vulnerable!!
You've just got to be carefull at the end of the day. I'm tempted to get a Caterham 7 at the moment, which by comparison is a death trap, but the handling is sublime, issue again is the only way to enjoy them is to drive them quickly, cruising around in one of these is a PITA.