Lots of people seem to think so: "This is why Windows Phone 7 is screwed. It needs apps. And to get apps, it has to compete for developer time and money with platforms that make money, either directly or by advertising."
This is an argument I've mostly seen touted by iOS fans, but as a long-time Apple guy myself I have some doubts. My iPhone usage consists overwhelmingly of just a handful of aps, most of all the Safari browser, the Email client, a Twitter client (current Tweetbot), Google Maps, iCal, Weather, Music, the Camera, and the Kindle ap. Most of these are just the basic stock elements of the phone. It's obviously true that if Microsoft can't put a good web browser, a good email client, a good media player, a good calender, and a good camera on its phone then it's going to be in trouble. But that's not a question of the size of the user base it's just a question of whether or not Microsoft's in-house developers are any good. Beyond that, the key things you need for a platform to get off the ground are cooperation from a handful of established firms—Amazon, Netflix, Hulu, and the like—who all seem to me to be willing to play ball. Beyond that, I dunno. Either Windows Phone's core functions + the hardware it runs on + the deals the carriers offer will be a good value proposition to consumers or they won't be. If they are, the user base will grow and you'll find there are plenty of good aps. If they're not, then it's doomed. But this all hinges on issues that are more ground level than the question of whether or not Microsoft can induce someone to generate a usable Instapaper ripoff.
The thing a lot of the commentary on the smartphone market seems to me to miss is that the vast majority of mobile phones in use in America today, and an even larger share of mobile phones in most other markets, are still non-smart "feature" phones. That means there's huge growth potential for the entire industry and no particular reason to think that the device side window of opportunity is closing.