No, Mac needs ZERO GROWTH. All they need is that, for every year, a significant portion of Macs sold are to folks that have NOT owned a Mac before. They can continue to sell 20 million with 10 million going to non-Mac users indefinitely. Every year, a few million folks will need/want an upgrade keeping the sales at the same level. Once people that have never owned a Mac have no desire to buy a Mac… well first that 20 million a year drops to 10 million and just continues to drop after that.
People who buy Macs buy iPads. The better thing to wonder is, do people who buy iPads buy Macs? Mac Users can increase unit sales of the iPad simply because the iPad is so inexpensive. Would iPad users be able to increase unit sales of the Mac? I’d say not likely just due to the cost difference.
I’d guess that there’s little significant overlap between PC users and Mac users on a grand scale just because there aren’t that many that NEED both. Any individual Mac owner MAY also own a PC, but it’s not just so they can have it around in case of emergencies, it’s because there’s a need for it, like gaming.
Any individual PC owner MAY also own a Mac… But, as a group of people, they’d be dwarfed by the millions upon millions that only own one or the other.