You're right, of course, horrible accidents are inevitable. However I don't think that'll impact the timeline much. If Burt were the only game in town, though, I'd be worried. NASA has gotten us used to an accident shutting everything down for a few years -- and when this happens to a private company, they too will certainly want to ground their fleet until the problem is found and fixed. However, the fact that there will be more than one player in this game changes everything. Although Mojave Aerospace is the first to reach space, there are at least four other fully-funded companies (Armadillo Aerospace, Blue Origin, Rocketplane Limited, TGV) that are now building flight hardware, with numerous other credible competitors which may get funding at any moment (most promisingly, SpaceDev). If any of these is grounded by an accident, the others will have no reason to stop flying, given the lack of commonality between them. And for the grounded party, the fact that their competitors are still flying will give them a powerful incentive to get back in the air quickly.

As for how isolated accidents will effect ticket sales, investment, and sponsorship -- I don't think they particularly will. Mt. Everest climbs are still popular, despite a 13% fatality rate, and formula car racing still attracts a lot of money, despite a high casuality rate. As long as the experience is exciting enough, people will buy tickets and sponsorships, and as long as it makes money, investors will invest.

Cheers,

Nathan.