"The combined revenues from apps funded by pay-per-download (PPD), value-added services (VAS, including freemium and subscription) and advertising is expected to rise from just under $10 billion in 2009 to $32 billion in 2015."
Apple, the most high-profile purveyor of mobile apps, has said it has seen 4 billion apps downloaded from its store, for a total of about $1.5 billion. For Juniper's estimates to bear out, other providers will have to come pretty strongly to the fore. This is especially true if the company's estimate that 85% of apps downloaded are free.
An additional issue is the relatively constrained area where smart phones are used with any frequency - industrialized countries. Even there, they are not as common as feature phones. Either smart phone use will need to grow or apps will need to adapt to feature phones as well.
Photo by Cristiano Betta