It turns out: securing on network services is a PITA that school network admins don't need more of; developing good ones is expensive and is best outsourced to a SaaS; on network/on device resources need more extensive vetting than web resources; on network/device resources take days to try out and roll out with cooperation of the understaffed IT team, web resources take one class and a link.

Skype is superceeded by a half dozen more targetted classroom video chats - none of which need installing or require the kids to have accounts.

From the perspective of more easily deployable apps that are more useful to end users, WebAsm > HTML5 > Flash. The fact the ad monopolies run our major web developments is convenient for them to piggyback these changes, but isn't the drive for them.