It is expected that Apple is going to make all of their apps on the App Store shared with all their devices. Apple is working on a project named Marzipan that can allow app developers to create an app for only one Apple device that will function across all of Apple's devices. When this happens, developers will be able to make apps for the App Store a lot quicker because they won’t have to consider all of the different devices that run iOS. Apple is going to accomplish this goal in multiple stages. According to Apple, developers will be able to directly port any iPad apps that they design to their Macs before the end of 2019 through the use of a new SDK that they are working on.
Microsoft has also attempted something similar to this in the past when they tried to make a Universal Windows Platform that can run on phones, laptops, and tablets. This attempt ended with them giving up on their Windows Phone product. Microsoft still supports universal software but they greatly encourage developers to add their apps to the Windows store instead of making one app that works across all of their devices.