Not necessarily. iOS already surpasses Android in mobile OS market share, and Android isn't making any inroads into tablets and other mobile computing devices, which will probably be the bigger market than smartphones.
EDIT: Also iOS growth is slower than Android growth, still, so it's just a matter of time before even iOS numbers get beaten, with or without Android tablets.
Not saying I disagree but I am saying you have to be careful with blanket statements like this. I recall Olympus stating that phones were simply incapable of competing with digital cameras, regardless of market. However the market has shown that phones eat into the low end market and the SLRs are pushing down from the top putting a lot of pressure on the mid-range market.
I could imagine that the 'phone' function as an adjunct to all tablets. All you need is a headset and voice dialing and your tablet can go VOIP/Cellular as the channel availability demands. Services like Gvoice making the endpoints number vanish by overlaying it with a 'logical' phone number.
Its not a huge step to cannibalization of the smartphone market by the tablet market.
It will be an interesting evolution.
That is awfully competitive even if you have a dumb phone and just a voice plan now. And is a market where iPhone can't even get close.