There's no sales tax at all unless you decide to buy it. At that point you pay all the taxes.
I don't think that's true at all. If anything, my initial deposit had taxes for sure. The contract also mentions that taxes are paid up front by t-mobile and collected pro-rata as part of the monthly payments.
The math in the rest of your post is questionable too. You're rounding numbers way too much in ways that's favorable for your argument.
After 1 year, you'll have paid 100+20*12+taxes which is above 350. If you upgrade at that point, you're essentially selling a 750$ phone (on which you've paid 350$) for 400$. Pretty sure in a year, you can easily sell an iPhone 6s 64gb for at least 500$ (which you mention to have done with the iPhone 6). Also, I don't know why you say your iPhone 6 cost 900$ (400+500). With taxes, it's really at most 800 unless you live somewhere with really ridiculous taxes.
Anyway, I think the only good part of this T-Mobile deal is getting the phone effectively 100$ off. Upgrading with "Jump On Demand" after a year essentially costs you about 100$, but I'd rather just go through the hassle of selling it myself if it makes me an extra 100$.