Why are you dealing in absolute values when discussing relative price comparisons? $3/kg raw materials costs is indeed many times more expensive than your typical mix of mild, high strength, and ultra-high strength steel structure with alumium body panels. Their average structural raw material cost is probably something like $0,75/kg at present.
And as for money saved on building new lines, remember early this year, Deepak stated that total depreciation on Model 3s was well less than $2k per vehicle. And it would of course be even less now.
Passenger car values don't decline linearly with age; they decline quickly, and then start to level off at a small fraction of their initial value. One may feel that that's not the way it should be, but it's the way it is. Extending the lifespan consequently has a minimal impact on TCO - in particular because those extra years are "way in the future", and thus highly depreciated via time value. You also have to extend the life of every other part of the vehicle to actually get more years, not just the frame.
I think you're dreaming if you expect insurance rates to decline dramatically (let alone halve). The car may not be readily scratched or dinged, but in an accident, it's still going to get damaged, and it's going to have full liability costs for in said accidents - and said liability costs might be even worse due to a less pedestrian-forgiving design.
Things like rust are generally significantly in the future, and thus heavily depreciated.
It's a simple matter of, if the Model 3 SR+ had a range of under 200 miles, and a LR had a range of under 250 miles, and both had correspondingly higher charge costs, and charge times, and shorter pack lives, and on and on... how much would that cost you in sales, vs. "well, it's tough to scratch and won't rust"? For a pickup? Sure, I'd go with the latter - "toughness" is the whole marking basis of pickups. For a sedan? No question, the former.
I compared to the Model 3 because that's the vehicle people have been cyberizing, and the vehicle for which we have an actual drag coefficient. But since Model X's drag coefficient has been alternatively reported at 0,24 and 0,25, relative to the Model S's 0,24, why would you expect a 0,02 gain for the Model Y vs. the Model 3?
Efficiency is a parameter that buyers hardly value at all. The knock-on impacts of efficiency, however, are highly valued by customers. These dramatic knock-on effects (range, charge times, longevity, operating costs, etc etc) are what has given Tesla their success in the marketplace vs. less efficient vehicles.
And efficiency is a major part of TCO. Efficiency is how much you pay for power at home, how much you pay on the road, and how quickly you rack up cycles on your (very expensive) battery pack, as well as how deep you're cycling said pack to go a given distance.