I think the worst case scenario is that they can't muster the money to continue on in the face of seemingly endless losses and setbacks.
Many companies think that the economics will work out for them once they've achieved a certain scale or years of efficiencies. It doesn't work out for all of them. The worst case scenario is that Tesla keeps losing money, investors lose faith in the company, they spend what money they have left, no one will give them more funding, and they close up shop selling the assets to another.
A more likely scenario is that the tech is years away, they spend a lot of money before the tech is ready, and then someone else who is better at manufacturing and distribution swoops in once the tech is ready and cleans their clock.
Right now Tesla has no competition and doesn't have to negotiate and compete on price; when they do, their margins will suffer and holes in their efficiency hurt them.
In the future, self-driving cars will decimate the market for vehicles. If we didn't have to worry about the issues around sharing cars, we might need a tenth of the vehicles we currently have. Even if Tesla becomes the Toyota of that era, it's 1/10th of what Toyota is today and it shrinks to less than half its market cap. Plus, with fewer sales, maybe they can't make themselves profitable, they fold up shop, etc.
Maybe self-driving vehicles will create really great mass-transit systems where you can hop on and off ad-hoc routes with well-appointed 10-20 passenger vehicles and get picked up and dropped off conveniently with little added time - probably a lot less time than our current situation with traffic.
This isn't to say that Tesla won't achieve its goals, just to point out that many things could happen to impact Tesla's future. It's not just about making an electric vehicle - Tesla has done that. It's about being able to effectively compete with a market that isn't going to stand still. If Tesla can't manufacture with the efficiency that other auto-makers can, they'll fail over the long run. Heck, GM had a lot of trouble keeping up with Toyota. Will Tesla become efficient enough to compete? Will the auto market dry up as autonomous vehicles come into play? Will we just get transportation from A to B via a transit network rather than owning a vehicle?
Again, this isn't to say that Tesla can't succeed. This is merely to point out that it's far from assured. If electric vehicle economics improve, competitors will come in and fight for those dollars which will drive down prices and profits. If autonomous vehicle technology becomes a reality, Uber, Lyft, and others will be ready to make the case that you shouldn't even own a vehicle and they'll be able to transport the same number of people with a fraction the number of vehicles.