It is not a question of doing worse, it is more for the sake of preventing a monopoly. Right now, 888 and Party are doing a horrible job of trying to steal business away from Stars after Amaya's recent unpopular decision s(this week's 2 + 2 podcast has some great insights into the whole scenario), but they could do some over the wall promos for say 1 month and help secure some new customers
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If Amaya buys Party, the nailed will be pounced in the coffin : who will put pressure on Stars? (not that anybody is at the moment, but Amaya would also secure the American market without a fight). Competition is the only thing to keep Stars somewhat honest, at this point.