Hmmm. The Cybertruck and the Semi are new vehicles, still in design, and without manufacturing machinery and assembly lines already committed. So I can see the feasibility of incorporating new battery cells in those. But the Model S design is set and the manufacturing lines are in use. The new cells will require new battery packs, potentially different cooling systems, and most likely different chassis architecture. I cannot see the new cells being in the Model S without a major revamp of the platform, and I doubt Tesla will do that for a low volume model like the Plaid will be. I could only picture that if tesla does a major revamp of the Model S (and possibly X?) architecture, inside and out, which sounds like a major investment. They would need to do all their testing all over -- including safety testing for all their sales markets. The Model S and X volumes are so low, it is hard to imagine that investment being prudent and worthwhile. They have not even converted S and X to the cells being used in Model 3, which would be a smaller leap (but still would be disruptive to manufacturing and would possibly require testing, I think).
Tesla has too much development on their plate -- Semi, Roadster, Cybertruck, and now the Plaid.
Frankly -- and as a Model S owner, I hate to say this -- I wonder if Models S and X have served their purpose and will not be further developed. The Plaid model is a sort of extreme stretch of the current platform. Maybe it will be the swan song...The Model S and X do not really fit with the high volume, low quality/low service model that tesla is currently operating. For continuing sales at high volumes, Models S and X require a different sort of buying/service experience, which existed before Model 3 but which is largely gone. I wonder if the pricier models are just not needed for Tesla's purpose to "accelerate the world's transition to sustainable energy..."
I'm pretty sure the Plaid tech destined for the Model S and X is probably the same as the Roadster. The Roadster prototype had two 100 KWh packs connected together. The Roadster may be designed using elements from the Model S, which makes sharing parts between the two easier.
If that is true, it makes sense to release the Model S Plaid and then the new Roadster with Plaid shortly after. Being a lighter car the Roadster will likely perform better than the S with exactly the same drive and battery hardware.
As for the Model 3 batteries in the S and X. I recall Elon saying that the S and X won't be converted to use the 2170 cells because they are unable to deliver energy to the drives fast enough to meet Performance specs in the S and X. It may be that with the larger cells made with the same build tech as the 18650s, they can't be cooled fast enough. The new batteries announced on Battery Day have a new way of building the cells that makes cooling more efficient so they can get more power out of a larger package quicker.