Bulb efficiency and bulb life are at odds. In railroad signals, the practice was to use low-power bulbs that barely glowed, and lasted basically forever(nowadays it is all LEDs, Iguess). I am sure no one wants 5W bulbs glowing deep orange at home. Targeting a rather low life is the same as targeting a minimum efficiency.
If this was the case they could simply have left it up to the market to decide the balance of efficiency and life. If your supposition was correct then the hypothetical "5W deep orange" bulb would not have sold.
Assuming that the collusion actually took place, which I don't put my bets in. I remember seeing 'long life lamps' on supermarket when bulbs were a thing, BTW.
