I think what you've written is pretty much what the "almost all programs have paths that crash" was intended to convey.
I think "perhaps the density of crashes will be tolerable" means something like "we can reasonably hope that the crashes from Fil-C's memory checks will only be of the same sort, that aren't reached when the program is used as it should be".
I think "perhaps the density of crashes will be tolerable" means something like "we can reasonably hope that the crashes from Fil-C's memory checks will only be of the same sort, that aren't reached when the program is used as it should be".