It's an interesting question, and one which must be considered by the authorities. But, in the end, I suspect they will conclude the negatives of putting canopies on single-seaters outweighs the benefits. The accidents we saw in the last week are very rare, whereas incidents in which drivers need to bail out of their cars quickly or need urgent medical treatment are more common. It's a balancing act and a perfect solution does not exists. Drivers are aware that, although the chances of something bad happening are quite low, there is still a chance.

Ayrton Senna: WDC 1988,
1989, 1990, 1991
McLaren: WCC 1974, 1984, 1985, 1988, 1989, 1990, 1991, 1998,
1999, 2007McLaren: WDC 1974, 1976, 1984, 1985, 1986, 1988, 1989, 1990, 1991, 1998, 1999, 2008