And in reality, the R1T and R1S share so much that it's really not like two independent lines for two independent models. They share motors, wheels, tires, seats, batteries, lights, console hardware/software, paint, etc. Sure they need to be assembled independently, but the supply of components going into each line is going to be substantially the same. Plus, they haven't really geared up the R1S production yet, so it's mostly R1T.
But "investors" are mostly irrational, so none of these "conclusions" about how much investors are "fretting" means much. Let's look at Tesla, for example. Rivian produced 1000 vs a target of 1200 for 2021. And that's enough to tank the stock? But not only did Tesla NOT meet their target of Cybertrucks for 2021 (despite Elon being the master of manufacturing, having learned the hard way through years of experience...), it's unlikely they'll produce any Cybertrucks in 2022 or maybe even 2023. And who knows what their range/price/speed/load capacity/etc. will look like at that point. But Tesla isn't taking a hit for that ... Irrational.
I think a lot of "investors" are spooked by their own shadow, and make drastic moves based on gossip and rumor rather than facts.
And I think that a lot of people, journalists included, are lazy about their research. They take what is given to them, rather than seeking out data for themselves. I see a huge focus on the Lightning in the media, and little or no mention of the Rivian a lot of times. Last fall I saw an entire 10 minute story on the Lightining on network news, with no mention of the competition, back when Ford was still planning on producing only 15,000 or so in 2022 (which is double what they had previously announced). I see this as Ford handing out PR packages to media, and the media rebroadcasting what they are given. Like a free commercial. Now, the Lightning is a story, because the F150 is the best selling vehicle in the US, but it's a shallow story, since it's so little work to say yeah, they're going to be able to sell quite a few Lightnings. There is no critical analysis - no one is saying wait, Ford is already offering a F-150 hybrid, but that's not selling very well at all - maybe the Lightning will be the same? I can't say either way, but I think there's an argument to be had that the main F-150 demographic is not composed largely of tree huggers who think meat is murder and climate change is real. In other words, not a demographic that is going to switch to an EV F-150 just because it is an EV. They are going to want to see that what they get is worth the added price. And there IS an added price. Best case IMO is that Ford keeps its market share of pickup trucks by making an EV F-150. If they didn't, their market share would be gradually eroded by competitors like Rivian and Tesla and even the 2 other big 3. There is no way that Ford will actually increase their market share significantly - the best they can hope for is to spend billions in R&D to keep their place on top.
I personally am willing to give Rivian a chance, because they've already done something that many have tried to do and most have failed to do - produce a brand-new vehicle with a brand-new company and actually deliver something. (I saw two in a parking lot just yesterday (they exist and are real), and I've test driven one - they are awesome!) Think of all the other companies that have crashed and burned before even reaching this point.