You probably didn't use Amazon's Android Appstore because very few people do. Outside of Kindle and FireTV devices, the Amazon Appstore is simply an afterthought; one that isn't as good as what you already have installed from Google. That's surely a big reason why the company is dropping the stand-alone version for "regular" Android devices. Come August 20, it's gone. I wish it weren't.
Not because I think you get a better experience from Amazon than you do from Google or because I can't find what I want on the Play Store. I hate to see it happen because it's a giant tech company with seemingly infinite resources that isn't Google.
