Amazon is practically giving Alexa devices away. This s because no one bought Amazon's smartphones, so they spun off Alexa into it's own product in the hopes people would use it to buy things from Amazon (they aren't).So i doubt someone that has invested that much money in lets say Alexa products and accessories is going to replace everything with SIRI compatible stuff.
Apple already has Siri on more devices than Amazon has Alexa on. iPhones, AirPods, Watches, and iPads all have "Hey Siri" and can control HomeKit devices.
Alexa isn't competing against HomePod, it's competing against Apple's entire product line.