My point, which you are obviously missing as well, is that the smartphone market is big, and growing bigger by the day. Look at how many smartphones were available 5 years ago...then 3 years ago...now today??
If your argument is "keep it exclusive to ATT because there exists ONLY a market of 26,880,000 (YOUR number of 16% multiplied by the other 3 carriers' customer base) customers who would buy a smart phone, then I hope you are not in a position of making large decisions at your current place of employment.
I don't think anyone saw the smartphone market being what it is now in mid-2010 way back in early 2007...so suffice it to say that the above customer base is probably going to only get bigger next year, and the year after. And also considering that between the time the first iPhone launched and today, there has been a massive amount of market parity...iPhone is not the only "awesome" touchscreen only smartphone on the market.
If the exclusivity had ended last years versus still going on right now, Android would NOT have passed iPhone OS to become the 2nd most used smartphone platform in the US. Period.