Launching on what, exactly?
Right now they have 9 launches planned using Atlas 5. That's not going to get them a usable fleet of satellites. If they want to launch anything beyond that, Blue Origin must finish the BE-4: a rocket engine years behind schedule. (Either using them for Vulcan rockets or for the New Glenn).
From Amazon's perspective, those 9 launches are money wasted if BO doesn't come through. I'd be ecstatic if they did, but my money is on yet another delay from them.
If I had to guess, by the time Kuiper is launching enough satellites to matter, SpaceX will be doing the same thing using Starship at an order of magnitude lower cost. And I don't even think Starship will be doing real launches for years.