They are wrong.
There are 3,000,000,000 cell phones in use.The iPhone is 0.2% of the cell phone market. If I want to text someone who doesn't have an iPhone (99.8% of the market) I have to use traditional SMS. But that's not the real problem. The real problem is that I don't know what kind of cell phones my contacts carry. That uncertainty will prevent me from trying to reach them via that channel.
There are 6,000,000 iPhones in use.
Being ubiquitous is powerful. Working on every device is imperative.
Rhetorical question: If Google launched a web app that only ran on MacBook Air computers with solid state drives would that be a good business move for them?