The only justifications I can think of for buying Blackberry are a: to rebuild them, b: to integrate their good tech into an existing system (a good play for an also-ran smart phone that wants more market share and is willing to gamble for Blackberry's old, loyal enterprise market), c: to deliberately put them out of business (a good play for a successful smart phone to either go after the vacuum created by their death, or prevent competitors from exercising plan B).
Is it such a stretch to imagine a buyer killing R&D and investment, and marketing the carcass for enough years of free cash flow to get back the $4.7 billion?
Value investors aren't necessarily growth investors. The margin of error they look for can come from a cheap price.
It's similar to, "What if AOL just tried to milk the subscriptions instead of blowing all that money?"
AOL has an ongoing cash flow from subscription payments. Those payments keep coming from customers until they either quit the service or die off.
Blackberry sells devices. They don't have significant recurring revenue streams - customers have to come back and re-up to a new device every 2-3 years, or they don't get any money. That means as soon as you're no longer offering a competitive product, the revenue stream can dry up very quickly. Especially if carriers and retailers decide you're no longer worth carrying.
Look at the 50% drop they've taken in the past two years. Even if that line just stays constant, you're talking about a decrease of 90% in 6 years total. It's rare than you can scale a tech business down 90% or more and still operate it in any reasonable fashion.