Well, the scenario for failure would be basically if the flow of money were to start drying up, and there wasn't enough left to finish the project. It is certainly possible that even the $90 million collected so far would run out. There is more money coming into the project pretty much every day, but if it were to happen that the monthly inflow of money were smaller than the monthly expenditures, then at some point cash would run out. Given the number of people working at CIG at the moment, the overhead costs and everything, Star Citizen easily burns through at least two million per month, and actually probably a lot more.
Keep in mind, the project is a race against time in this regard. People are very happy to back Chris Roberts. But if the project were to stretch on for too long, then the number of people willing to put more money in would start falling. At the moment, this doesn't seem like a serious risk, but I suppose if 2016 were to come and go without a release, things might start be a bit less comfortable.
Of course, even then, Chris Roberts might be able to find a way forward. A traditional publisher or some other form of investor might be willing to step in and help finish the project, if it is promising enough.
By the way, there was
an article about Star Citizen and crowdfunding on Gamasutra today, with a very interesting graph showing, year-by-year, a comparison of total funds raised by Star Citizen and the total funds raised by
all game-based Kickstarter projects. Amazingly, in 2014, Star Citizen actually raised more money than all other projects combined (but it was a very bad year for Kickstarter, with no big projects on the horizon). So, it's a very remarkable result. It's also interesting to note, however, that based on the data for 2015 so far, this year Star Citizen will probably raise a bit less this year than last (a very rough projection would be about four-five million less). All in all, things are looking great for Star Citizen, but I definitely wouldn't say failure is not an option.