Given how long it took the general populace to let go of Windows XP, I predict a pretty similar turn of events (or rather lack thereof) with Win10. By and large everyone's grandma and parents and auntie will just keep on using their same old computer as it is, possibly eventually turning into a petri dish for every exploit and piece of malware in the known universe in the process.

The majority of casual home users will throw away their computer and buy a new one only if it stops working or possibly if some new piece of software or more likely some future web site won't work with it. Otherwise, to most non-nerd users it's just an appliance.

Office and corporate deployments are another thing, but OS end-of-life situations are not new to any of those guys.