Submitting to more than one app store has a very low marginal cost to the developer
Except that it doesn't. Assuming you are moving to the Amazon App Store...
Does your app provide notifications? Need to rewrite it using the Amazon api.
Does your app use maps? Need to rewrite it using the Amazon api.
Does your app use AdMob advertising? Need to rewrite it using another api, e.g. Facebook.
Does your app use in-app billing? Need to rewrite it using the Amazon api.
Does your app use Play Games to provide multi-user, cloud saves, leaderboards, etc? Need to rewrite it using the Amazon api that doesn't provide the same features so you'll also need to roll your own version of them.
Does your app use the modern location api? Sorry Amazon don't have an equivalent so you can't sell your app outside Google Play. Alternatively you can rewrite it to use the old battery draining location api and have users complain your app literally sucks (their battery).
There are more examples of features in Google Play Services that are required by most apps that makes porting them to Amazon time consuming and difficult or impossible in some cases.