Good question, Artur. I don't know really, but here are some guesses.
1. FL will probably last three years in reality, as 2nd generation won't be delivered until late 2015 or early 2016.
2. FL will have enough differences to attract new customers, generating at least some sales income compared to dwindling sales of first generation. My guess is that Porsche knows pretty specifically what this new income will be.
3. FL allows Porsche to continue to recoup its investment in the basic components of the first generation car, while at the same time evaluating new components that will ultimately make it into the new generation.
4. Porsche doesn't offer special sales and discounts on its models, so FL is a way to generate new interest and income on basically the same product.
5. Facelift is really a misleading term, which gives the impression that the car is simply a slight design makeover. Engine, suspension, lights, bodywork and interior and bodywork improvements are expensive and more substantial than we suppose casually.
6. Customers in the long run probably appreciate "gradual but high quality Improvements" over a model's history, as opposed to waiting four or five years for a "new" car.
Just some thoughts, and I welcome other comments from Rennteamers...