The existential FEAR of the smartphone ecosystem players (Apple, Google) is, that another ecosystem (!) may come along, one that is tighter integrated into the daily lives, is more predictive of the users' needs, requires less interaction and is not under THEIR control.
Because this is not about devices, it's about owning the total userbase of that OS-ecosystem.
Replacing the Smartphone has been attempted numerous times in the past decade, but no device was able to replace it as a consumption device. Now technology has reached a level of maturity that Smart Glasses may have a shot at this. AND they come along with their own ecosystem as well.
Whatever happens, they won't replace all phones within 5 years. But it's possible that such a device would become a companion to an iOS/Android phone and within 5 years gradually eases off users of their phones into that other ecosystem.
And that's scary for Apple and Google.
Because this is not a device-war, this is an ecosystem-war.