It’s hard to say. My guess is as Amazon makes their product line obsolete, that’s when people will look to either replace the current device or make it a secondary device elsewhere in their house and then upgrade their primary one.
Brand loyalty is huge in this use case. Unless people know that there are alternatives, my guess is that a huge percentage will just stick with Amazon. That’s why it’s so important to be a first mover. It’s also a reason why Google has a chance.
With so many people on Android about to experience how well Google’s Assistant is (many of the Android phones don’t have it, but people upgrade their phone every 2–3 years, so most Android users will have it within the next 2 years), it’s likely that people will want to get the additional benefits of it being always on you (phone) and at home (Google Home) and get they synergy that that will provide. That’s an area that Amazon fails at, and is Google’s opening.
There’s a very small opening for Apple here, too. But only if they can do a massive upgrade to Siri. Apple fans are loyal fans, but Siri has a long way to go to compete with Google and Amazon.
So, to very verbosely answer your question, I don’t think that people will just one day get rid of their Echo and go with a competitor, but I do see people getting tired of their Echo no longer being able to satisfy their needs (one to two years from now), and Google being a contender because many people have seen how well it can work on their “new” Android phone.