"never individually bought a car that cost more than $15k" You must be buying used or just buying the absolute cheapest new cars on the market. Don't compare used cars to new cars. You're absolutely right about the depreciation. The smartest thing to do financially is buy a 3 year old car with low miles. It's nearly 50% cheaper, but still practically new.
Now that that's out of the way, let's talk apples and apples. Looking forward to the Model 3, assuming a $35k MSRP, the 5 year TCO should be lower than a $35k BMW or something else in that price range. Why? $1,000 or more saved on gasoline per year. That's at least $5k over 5 years. If gas were to go back up to $3 - $4 per gallon, that would be up to $10k in savings. Reduced maintenance. No oil changes and air filter changes. While these aren't terribly expensive ($200-$300 per year), they're a nuisance. I value not having to carve out slots of time for oil changes.
Early adopter. Being an early adopter is never the financially prudent move. But when you buy your $10 DVD player from Wal-Mart, you should be thankful for the early adopters who shelled out $1,500 for the first DVD players because they made your cheap DVD player possible. Same with EVs. in 2010, battery costs were $1000 / kwh. Today, 6 years later they're under $200 / kwh and more advanced to boot. They're projected to fall under $100 / kwh in short time. This is thanks in large to the high volume producers/consumers of the "expensive" EVs.
There's nothing wrong with being financially prudent, but don't think just because your financial math doesn't work for you today, the math is "bleak" for EVs. And when you're buying your $15k used Model 3 in the future, remember to be thankful to the Model S/X and early Model 3 owners who made it a reality.