In practice I wouldn't expect many devices to have lockdown mode turned on, and the people who are turning it on probably aren't also using the same device to play Fruit Ninja in a browser. This is a feature explicitly designed for people who have reason to believe they're being personally targeted by national intelligence agencies, or other extremely well funded organisations.
<Insert rant about how I miss my Windows 8 phone because it had less crap on it here.>
The only thing I saw in the writeup that I can imagine normal people over 25 missing is web font icons, and maybe emailing PDFs around to sign with iMessage. (Though those come in as jpegs from cameras or PNG screenshots half the time anyway...)
The blog says "Should You Turn it On? Yes. Seriously. Turn it on when you have a supported OS and don’t look back." If that becomes the general advice, I imagine it will end up getting more broad use - even if most of the people who turn it on don't really need the extra security.
I am writing to a somewhat technical audience on my blog... but, yes, I don't care if my devices can't play some online WebGL game if the tradeoff is far better security in general.
Also, since you can turn it off for specific domains, it's easy enough to re-enable WebGL for some site, while still having Lockdown mode apply to all the random ad serving backends and such you come across. If you're not someone who might be specifically targeted, I think that's entirely reasonable. Secure by default, drop the security level somewhat, by concrete actions I've taken, for some site I want to do something more on.
At some point, I'd assume attackers will try to get people to turn it off so they can attack, but you've made an awful lot more noise by that point.
