Case in point: new cell lines were needed for the 3/ TE, so they came up with an optimized cell size. S/X continued using the existing production infrastructure.
True, but I strongly believe S/X will start to use 2170 cell packs, once the production rate is there.
You're second-guessing Tesla and fretting for the sake of fretting. Do you think the battery pack is ahead of everyone else (Sandy Munro and Jack Rickard seems to think so)? If so, then what got traded off? Because they're ahead of everyone else with the densest pack at the lowest cost.
If not, then they that's what they've traded for cost, which goes counter to what you're alluding to.
The battery was designed to a certain spec, which is made out of maybe hundreds of parameters like cost, weight, storage capacity, rigidity, material availability, etc. Getting those absolutely right, for a new component of such complexity is simply impossible – there are just too many levers.
So here's what you usually do for the initial design: You overspec the critical parameters (eg. safety and storage capacity), while giving others (eg. profitability, etc.) a lower priority. After all, your top priority to get it out ASAP,. Everything else is pretty much highly educated guesswork.
If your lucky and did your job as best as you could, you'll still end up with a breakthrough – like in the case of the M3 pack.
BUT here's the kicker:
As I said before, true profitability – not being 15%, but 50% cheaper than your competitors – often kicks in with later revisions, when they know what fat to trim and what parameters to adjust.
To me it is still unclear, how they'd do pack revisions without affecting the much needed 24/7 production. Major revisions could easily result in multiple weeks of downtime for a production line, and they obviously can't just add new lines for every revision. Even updating lines one-by-one would be a logistical nightmare.
Now, having a breakthrough product at hand doesn't mean it's future proof. As we speak right now, there are hundreds of people around the world reverse-engineering the M3 pack. Let's say, they find some fat to trim and combined with their own expertise, they somehow manage to come up with a pack that's 20% "
better" (read: lighter at same rigidity, cheaper, etc.) than what Tesla has now. So when they start to build those reverse-engineered M3 v2 packs in eg. 2021, it's in Teslas best interest to counter those with an even better battery.
R&D wise, TSLA can do it, no doubt. But how do they plan to actually pull those revision-cycles off in the factories is not clear to me. I'm happy to hear everyones thought on this.
That's why I said battery pack, not cells. Further, your claim that the Model3 pack design did not come about from optimizing designs from the S is plain ridiculous.
The M3 pack is as much a MS pack, as the iPhone 4 was an iPhone 3GS.