The fact that most of the time market leaders resist cannibalizing the main source of their income is well known. The reasons are complex and connected to the nature of large human organizations. And it is also why most truly disruptive innovation is driven by smaller companies.
This cannibalization argument is the same reason that Henry Ford only sold black cars.
Car companies didn't (actually, if you go back long enough ago, they did, but they're over it now) have to transform their company and manufacturing processes in radical ways when they began offering both types of fuel, any more than had to when they began offering multiple cylinder displacement options (2.0L, 454cc, etc.) Diesel and gas engines both require fuel systems, radiators, alternators, air intakes, exhaust pipes with mufflers, transmissions, etc. It is actually possible (although not always easy) to just replace one type of engine with the other in a user-owned car, so much more in the factory.
It is quite another thing to rip out everything involved in the motive function, except the axles, wheels, and tires. If BMW wait too long to start the process, they won't get the results to a BMW quality level for many years. They'll have to decide whether they want to saddle the brand image with crappy electrical cars or if they'd prefer to sell their crappy electrical cars with a low-prestige marque.
