Yes, it did indeed look like a nasty accident - but it wasn't Morgan, and it was also a comparatively old car, so could well have been caused by a component failure - something to which a modern Morgan would not be exposed in the same way.
Given that the components most likely to fail and cause an accident are the old style running gear rather than the engine, I cant see that, but I have to agree 100% with the rest of what you say.
I'm not trying to pretend that a roll bar is useless in the event of an accident in which the car ends up inverted, but I am trying to point out that
a) Those kinds of accidents are incredibly rare
and
b) In the kinds of accidents that do occur much more frequently, especially side or three-quarter impacts where the car doesn't flip, it's possible that a roll bar will cause additional injuries to the head if you're not using a proper 4 or 5-point harness and/or wearing a helmet.
The last thing I would ever wish to do is to put anyone off the idea of driving a Morgan, and if you feel that you still want a roll bar then go for it, but I'd like you to be reassured that many thousands of Morgan owners drive millions of miles quite happily without feeling the need for one - and that certainly doesn't mean we bimble about like we were driving Miss Daisy!
In your situation, where you'd need to have full type approval for any modification to your car, I would seriously recommend that you review the benefits (enhanced survival risk in the case of an incident with a vanishingly small probability of occurrence) compared to the costs (inconvenience, greater risk of head injury in normal driving).
When you are rattling around inside the cockpit during an accident, what you need is soft squashy stuff and not a steel tube next to your head. And if to fit inside the hood it ends up being too low to function as an roll bar anyway, whats the point?