what about if they added another couple of wheels to the bike for stability and then cocooned the rider in some kind of seated cockpit to protect him during accidents?
I know you're joking, but one year whilst I lived there in 2011/12ish, they had a sponsored lap with an Audi R8 I think it was. The driver nearly binned it
badly at the first slight bend at the bottom of Bray Hill. Which is basically a few hundred metres from the start line.
The fact is, the roads of the course are pretty-well maintained compared to the rest of the island, but they're still roads with grids and bumps and tree roots and shit and it obviously doesn't take much to throw a bike off, or indeed a car. As others have said, they tend to be lined with walls and houses.
In a way I am surprised it carries on with relatively little clamour for it to be stopped, but if you speak to any of the riders they'll all say the same: they know and accept the risks.
One thing they could do is up the criteria for entry. Of course very experienced riders have died, but it does tend to be the less experienced that crash more often.