Actually, there are lots of reasons to doubt that. Wall Street has this microscopic vision when it comes to Tesla, interpreting every cough or blink of Musk's and every change in personnel as if the company's future was at stake each time -- I cannot remember any company undergoing that level of scrutiny in the financial world. (If Wall Street had paid the mortgage industry just 1/100 of the attention they are paying now to Tesla, which in terms of capital represents far less than 1% of that mortgage industry, the crash of 2008 would never have taken place...). Wall Street has been wrong on Tesla at every turn so far, just like it has been wrong on Amazon since it was founded...You do realize that Tesla is in serious financial trouble, right?
That Tesla is having a cash crunch at present is almost certainly true, but it's hardly a surprise, as it has to do a huge build up in Shanghai (the Gigafactory building will be ready for setting up the production lines in 6mos, so Tesla must buy a lot of very expensive equipment) and another at Gigafactory 1 (and there they have to continue adding to the current building, which is only 1/3 complete, so they have both construction costs and equipment costs); in addition, they need to get started on planning a production line for the semi and they have a bubble propagating down the production-payment-delivery line due to their starting shipments to Europe and China, with long transit times for the vehicles on the ships.
But, and that's the key, it's all investments in battery and vehicle production, it's temporary (the bubble will disappear in a month or two, when shipments to other lands are routine, and the buildup in Shanghai will be over by end of year) and they still own the EV market and will continue to do so for the next couple of years at least -- the only real competition coming up before 2022 is the Porsche Taycan, which will ship around New Year and will probably take some sales away from Model S. (VW is getting close to something real, but their first vehicle looks like it will be a Golf-type and will most likely be aimed at Europe. The new Nissan Leaf is quite nice, but overpriced in comparison with the base Model 3, and not made in large volumes. Very little is known about efficiency for the forthcoming vehicle projects -- but so far nothing has come even close to Tesla.)
Assuming their expectations for Model Y pan out (again, there is no competition for it for at least 2 years), Tesla will have plenty of time to recoup their current investments.
If Rivian produces another niche vehicle for rally racing, that's great -- the more BEVs out there in the press, on social networks and TV, and in the streets, the better. But it's not Rivian that's driving the scene; at this point, if Tesla were to fail, Rivian would fail very soon thereafter. The real risk is that various propaganda machines set up by the likes of the Koch brothers are out to discredit clean energy in general, BEVs more specifically, and Tesla in particular, and if they can convince Wall Street to drive Tesla's stock down by 80%, they might just succeed in killing Tesla, delay wide adoption of BEVs by 10 years, and almost certainly kill our civilization's last chance at survival... (Europe will try to pull ahead on the energy front anyway, but it is increasingly disunited; and nationalist types, on the ascendant in Europe just like Trump has been in the US, never seem concerned about the future...)