Why would it be too late? This isn't a social network where the network effects make competition almost impossible. Apple already has a platform in HomeKit that leverages the much larger iOS installed base. More HomeKit compatible devices are being released all the time.
In fact many people criticized Apple for imposing requirements around hardware-based security which slowed the release of devices, but the recent DDOS attacks and the Internet of Shit Twitter account has shown that was the correct move.
I don't think there's any deadline for Apple to release an Echo competitor. If they release a product that is better/differentiated in some way, people will buy it. And they'll likely have a number of HomeKit devices already that work with it.