It's bigger than you think. The problem is that it's too easy for a company to set up any modern smartphone to access company servers, retrieve/view/edit data, and provide decent security. So, anybody who wants their iPhone or Android device to load their work email can do so, albeit not as well as a BlackBerry. Basically, RIM won that market, but then that market got more options. It now comes down to the cool factor, which isn't RIM's strong suit anymore.
But the primary buyers of smartphones now are young adults. RIM needs to capitalize on this new market, because the enterprise market alone won't keep them alive forever. It's like you said, people have more options now, and the smartphone market has expanded greatly since Blackberry pioneered years ago.
It's similar to the gaming industry. Nintendo (with Wii and DS) expanded the video game market past the "core" gamer demographic. Being complete caught off guard, Microsoft and Sony frantically tried to capitalize on this expanded market. Microsoft got it right with Kinect, while Sony stumbled with the PS Move.
And even Apple (with iOS), and Google (with Android) expanded the mobile gaming market to a wider audience. Again, Sony tried to capitalize on this new market with Xperia Play and PS Vita, but they stumbled.
In a nutshell, RIM is the Sony of the smartphone industry.