I don’t think that Google or IBM stand a chance at dethroning Alexa. IBM barely has a horse in the game (and never has had a footprint in retail with mainstream consumers), and Google is a decent competitor but is relatively agnostic for users. Search and Gmail are not enough to proliferate usage for them to be a market leader.
Where Amazon shines is in the retail space. They are the swiffer of the web. Nearly everyone that has an Amazon account makes at least one purchase per month. Friends and family use the same accounts often to take advantage of prime. Once the video calling features takes hold when the Amazon Echo Show is released the gloves are off. That will be the tipping point where Google, Apple, and IBM won’t be able to keep up.