The definition of a disruptive technology is one that starts off objectively inferior but which, on projected technology trends, will meet the market need at some future date. The existing market can recognize these facts well in advance, but when the time comes to switch technologies finds upstart incumbents using the new technology with a cost structure the established companies cannot match.
Electric cars qualify because they do not at the same price have the same range, acceleration, or refuel time as gasoline cars. But all three aspects are improving exponentially over time. On current trends, electric cars will be comparable to or better than gasoline cars on all three metrics within 20 years.
What Elon has done is realized that the cost difference between building a top end car and a bottom end car is not that great, and at the price of a top end car he can deliver equivalent to better acceleration and range, with recharge time that is acceptable to many. This is very unusual - normally the upstarts in a disruptive market start at the bottom of the market and move up. (Indeed that was the pattern that Christensen predicted for electric cars.) But the mechanics work the same way. By the time Tesla can deliver a mass market car at a mass market price, the incumbents won't have the technology or business structure to deliver a competitive car at a competitive price.
For all of the reasons why existing companies will not be able to modify their technology and business structure to what is needed to compete with Tesla, I highly recommend Clayton's followup book, The Innovator's Solution.