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.
What BMW doesn't see is that Tesla is gunning for the 3 Series market with the GenIII car. Tesla is confident they can make a substantially better car than the 3 series (or Lexus IS, Audi A4, etc) in most regards and make it priced competitively. I don't think BMW is taking Tesla seriously... they're more focused on existing competitors.
If Tesla succeeds in making a substantially better car than the 3 Series that's priced competitively, then everybody in that market (BMW 3 Series, Mercedes C, Lexus IS, Audi4, etc.) will be in catch up mode. And that will the story of the decade in the car industry... how a scrappy Silicon Valley startup did the impossible to the auto industry.