> For finance etc., I'm not sure locked doors & frosted glass are sufficient if they're embedded with other offices and there's lots of intermingling foot traffic in the area, but I suppose with relative few additional precautions it could work
I know for certain that shared office space for lawyers is common. I actually visited such a space just a week ago. This particular space was hideous and depressing, but many 1-3 attorney law firms were operating right next to each other.
At least for law, there are certain ethical obligations everyone needs to follow, and if you break them, you can lose your license, get fined, and in extreme cases, face jail time. For security, in addition to frosted glass and locked doors, maybe WeWork could devote certain areas of a space to certain professions, i.e., this floor is for lawyers, this one is for accountants, etc. Only the worst and most unethical lawyers would break into another law firm's records, and there are plenty of existing processes for dealing with that.
The big point here is that with a bit of tweaking, WeWork could service just about any office-based company. There is a currently small, but fast growing demand for the type of office culture they are promoting. It's plausible that in 20 years, the "startup" style workspace will be the norm, rather than a niche millenial perk. It's not clear if WeWork will ultimately succeed, but they have a huge head start and seem to be tapped into what a quickly growing segment of office culture is demanding.