I was questioning your analogy with Apple. I'm sure they could win the low-end; but they don't want to.
Addressing the question of whether Musk can win it... well, firstly it is uncertain (and he might get hit by the proverbial bus), so we have to factor that in. But, some first mover factors in his favour: he's further along the engineering learning curve than anyone else; and further along winning market headspace; and further along getting every little business thing right to make it all happen. That is, he has a high probability of succeeding.
Secondly, I think Toyota or General Electric are better comparisons than with GM (whose cap reflects its terrible shape), which are 5 times bigger. (see the figures in my other comment: https://news.ycombinator.com/item?id=6567057) That is, the potential prize is larger than given in the article.
Whether Tesla is over-priced or not I couldn't say - except that the market overreacts to both good news and bad news, so it's probably best to wait for a dramatic stuff up that doesn't actually affect its real business prospects, and buy the depressed stock.
The question isn't whether or not Tesla will make a play for the low/mid-end market, it's whether or not they can win it.
The consumer car market is very different from the luxury car market. It's a world where it's not enough to execute well, but also do it with razor thin margins. Tesla has proven their ability to concoct grand ideas and bring them to life when money is no object - it remains to be seen if they can do it to the tune of $20-30K, or even lower.
Tackling the high end first was absolutely the correct strategy, but now the market is heating up. Are we so confident that Musk can lay waste to every single major automaker out there that people think they will be worth the entirety of Fiat, or half of GM?