There are perhaps many devices that COULD be subject to attack, but many exploits are rather more dangerous 'on paper' and require a specific set of circumstances, possibly even direct access to a device, to enact.
As such I don't think too many people need worry, whether they have an old Android phone, Windows Phone or even an iOS device (not immune either).
Those people who have a really old, insecure, phone are most likely not presidents and are not at any real risk because they probably rarely use their device. If they were heavy users, they'd likely upgrade anyway because chances are their battery is now on its way out, and they'd like a better screen, faster data connectivity or whatever.
Many phones in the last 2 years or so will get security patches even if no more OS updates, and as I've said before, Play Services and other Google apps will be supported for many years beyond the ending of OS updates.
Would I like it for phones to get OS updates for longer? Sure.