Personally, I think you're correct that Tesla will eventually move into the large global market for sub-compact cars at the $15K price point.
Firstly, a "lower cost vehicle than the Model 3" was mentioned specifically in Master Plan - Part Deux: (published on Tesla's blog on
July 20, 2016)
Master Plan, Part Deux
Expand to Cover the Major Forms of Terrestrial Transport
"Today, Tesla addresses two relatively small segments of premium sedans and SUVs. With the Model 3, a future compact SUV and a new kind of pickup truck, we plan to address most of the consumer market.
A lower cost vehicle than the Model 3 is unlikely to be necessary, because of the third part of the plan described below."
Later, Elon twice hinted that a compact Tesla could be coming in 3-5 years in comments he made during the June 2018 AGM Q&A session and his August 2018 video interview with MKBHD. This likely refers to a hypothetical $25K 'Model 2', to be introduced no sooner than 2023.
This last part
"unlikely to be necessary" (highlighted from MP-pt2 above) refers to the successful wide deployment of FSD/robotaxi. However, these products are NOT mutually exclusive with cheaper cars. I would purchase FSD with a sub-compact Tesla "Model 1" due to the extra utility. Elon has hedged his bets, and left open the possibility.
The remaining unknowns, the technical feasibility, is perhaps the easiest. Indeed, a version of Moore's Law is almost certainly at work with EV battery costs, and with Tesla's manufacturing abilities (due to robotics).
A 200-mile rge light-weight city car would NOT require a 54KWh bty like the 3SR+ (which has 20% more rge than that). Indeed, the SR+ itself appears to use 122/148 KWh/km or ~18% less than the 3LR rwd, while only having the lower curb weigh in its favour (as per Bjorn's early testing)
A 27 KWh pack and sub-compact dimensions easily put a
"Model 1" into the required 85 Wh/km consumption target required to achieve a 200 mi rge rating. 'Maxcells' of 5 years from now could be approaching 500 Wh/kg while retaining Model 3 LR charge rates, life cycles of 2x current products, and costs of 50% measured by $/KWh.
Do the math, and that's a sub-120 pound battery pack that costs $1,350 at wholesale. That's about 11% of the wholesale cost of a $15K retail car that still retains a 20% margin. The secret sauce for more profits? If needed, targeted averting / SaaS leveraging that large screen and their captive audience.
Ultimately,
it is up to Toyota et.al to choose to compete, but to date we've seen no signs they're willing to produce a compelling compact EV, or to even
TRY to retire their gasoline-powered Corolla line. For this reason alone, and the simple fact that FSD will be equally compelling on a 'Model 1', I believe Tesla will produce a $15K compact car by 2030. And I believe that product will eat the lunch of any large automaker of gasser sub-compacts who refuses or fails to produce compelling EV alternatives in volume.
Cheers!