Was the exclusivity to get AT&T to meet their terms? To get them to agree to their subsidized prices? If so, you have to ask whether it's really about market share or profit per device and controlling your ecosystem.
When the iPhone first came out, it wasn't subsidized. At least, not in the normal manner at up front sale time.
Apple initially sold the phone at a high price for a nice profit, and then ALSO took a monthly fee for each customer... money which usually was put aside for a subsidy for the customer. Basically, Apple was double dipping at the expense of the user.
That business model couldn't last, however. Partly because Apple had to radically drop the price to keep selling, and partly because so many phones were ending up on other networks that didn't pay the monthly fee.
But I digress. The exclusivity came about because, by mid 2006, Apple had no big carrier partner yet. Verizon had been turning them down starting back in mid 2005 (understandable with no product to see), and AT&T (Cingular) had made no move either, even though they had known about Apple's phone plans since early 2005.
So AT&T (Cingular) was in the driver's seat in many ways. It was no extra skin off their back to give the customer subsidy to Apple on a monthly basis, and they didn't have the thriving app/ringtone business that Verizon did at the time. So they basically gave up nothing in return for Apple agreeing to a long exclusivity. It was win-win for AT&T.
As Cingular put it right after the iPhone was shown off, and Jobs got
on their nerves by claiming to have dictated to a carrier:
"I'm not sure we gave anything," (a Cingular exec) stated. "I think (Apple) bent a lot."
Still, it's hard to have a "worst decision" for a profit machine like the iPhone.
Yeah, but think how different things might be if Apple hadn't given Android four years of total freedom from competition in over half the US market.