Number of Competitors
If you're going to do a BCG matrix, be sure to list more than two competitors. As in your example, Pepsi will definitely not be a "dog" if you'll just include more participants in your grid. Personally, a "question mark" product is the one who gets the second largest budget (behind the star product) and is the closest thing to become a star in the future, given the right promotion, rather than a cash cow product, given a few more years, will start its decline towards the dog quadrant. Just my perception. Cheers!