We're not talking about evolutionary change but revolutionary. Drop in parts number is so drastic that it allows for more competitors to sprung up (hence Tesla)
I'm a cost accountant and I do this sort of stuff for a living. You have the cost accounting completely wrong. The different in part numbers provides Tesla no cost advantage at this time because the parts they have to buy are significantly more expensive. Electric vehicles have such low sales volumes currently that any cost advantage they might have from reduced part counts is hugely swamped by the high R&D costs and fixed costs of production. They simply don't have enough volume to reach minimum efficient scale.
The risk for established players is in going from oligopoly and into a commoditized market.
There is minimal risk of automobiles becoming meaningfully more commoditized than they already are. Switching to an electric platform will not change that. A commodity product is one that one unit is indistinguishable from another. That does not describe the car industry unless you abstract more than is appropriate. The established players you are talking about already have the capability to develop and sell an electric vehicle. Several of them have already done so. Nothing Tesla is doing is outside of the big automaker's capabilities. They are staying out of the market because the market simply isn't big enough given the state of the art in electric vehicle technology right now to make it worth their while. There is enough room for a few niche products but that's it for the time being. It's not worth their time right now because they cannot make a profit doing it yet. Even Tesla hasn't made any sort of meaningful operating profit on car sales yet.