I'll respectfully disagree, as I think the shift had more to do with the fact for most users, everything changed at home.
For years, most people had a cheap candy bar / flip phone, or at most, an expensive candy bar / flip phone. All phones were pretty dumb and very similar. Remember the RAZR? It was The Thing for a while, but looking back, it wasn't really that much different from everything else.
The hardware was sexy, but the software was horrible. Nobody liked the OS, nobody thought the phones were responsive, etc. It's essentially what the more vitriolic anti-Apple folks claim to be true with the iOS ecosystem. Except that in this case, it was true. There wasn't much to redeem the phones beyond the case. It was exactly the same garbage people had been force-fed for years.
At the same time, millions of people had BlackBerry phones provided by their employers. They offered email that worked and calendaring that wasn't a step below some CP/M program from 1980. (Seriously, did anyone actually use the calendar applications on those old consumer phones?)
While it was great to have email anywhere, it wasn't enough to shift the market away from the run-of-the-mill consumer devices and over to smartphones. At least not en masse.
Then the iPhone arrived. Suddenly people had phones that did a whole bunch of things people wanted to do and did them a whole lot better than their corporate-provided devices. While it had weak support for the features corporate IT demanded, it was immensely popular on the home front. The candy bars and flip phones got wiped out.
Customers went from owning a terrible phone for personal use and a fancy phone for the office, to owning a fancy phone at home and one at the office that seemed - quite suddenly - rather archaic. Rock solid, but... quaint.
Then Android hit the market. The iOS app store opened. The momentum had firmly shifted to the consumer side of things.
BlackBerry was lethargic in responding. Famously, they reacted to the iPhone launch with disbelief. They literally believed the feature list was a lie, so they didn't worry about it.
Even after reality hit them, BlackBerry's handsets were just more of the same. They admitted to not even knowing how many models they were making. Their tablet didn't even have email. In 2010. It made them look ridiculous. Especially in light of the fact email has always been BB's bread and butter.
Meanwhile, iOS and Android kept improving their corporate IT support and allowing third parties to rollout all kinds of management solutions without interference.
Executives carried iPhones and Androids and started wondering why they had to carry two phones.
Corporate IT people did the same. Sure, they were annoyed that they couldn't get the same crazy granularity in security on non-BB devices, but BlackBerries were looking more and more like the typewriter stuck in the corner of the office. It worked, but you didn't use it unless you needed to... and why was that thing still even around anyway?
BYOD was the final nail in the coffin. Penny-pinching execs eyes lit up when they saw the possibilities of not having to buy handsets anymore. Their iPhone and Android using employees could simply buy their own phone, with their own money!
BlackBerry's decline was due to a fundamental shift in the way people use phones and the failure of the market leader to recognize that fact until it was far too late. Had they mobilized on Day 1, I think they could have rolled-out BlackBerry 10 by the end of 2009. In which case, things probably would have been far, far different.
Honestly, it kind of reminds me of the home computing wars. The IBM PC arrived in 1981 and wiped out the entrenched CP/M market within a couple of years. Apple showed up in 1984 with an entirely different approach and snatched-up its own sizable segment. Commodore and Atari rolled out their own new platforms with strong niche appeal a bit later.
For years, Microsoft and Apple jostled for double-digit marketshare, while Commodore and Atari scrambled to claim whatever was left in their wake.
The smartphone market we ended up with Apple > Android > BlackBerry > Microsoft. The first two being incredibly strong, while the latter are struggling to justify their own existence.
The big difference here is that in the home computing wars, what people used at work eventually ended up being what they used at home. Here, it's the opposite.