Ask an entrepreneur 20 years ago what platform they were developing for, and you might hear 90% - Windows - 10% Mac. Asking the same question 15 years ago, you likely heard 75% were developing for/on the web, less than 25% for the desktop market. Fast forward to today, most startups are developing for Mobile first, and of those many choose IOS first, and eventually Android. Many of these Mobile first companies even remain Mobile only, never even considering the web or desktop market.
I believe within just a few short years (if not now), many should be considering Cloud first. By architecting as much of your product to exist in the cloud you immediatly make it a much easier, and less expensive challenge to enter your next market. So today, you might be launching IOS first, because you might only need a UI Shell (If you run in the cloud) to launch Android it may or may not make sense for you.
HOWEVER, the point of this post is that you want to plan for tomorrow, not today. And the tomorrows of the world are filled with new platforms. With 5G coming and a plethora of new platfroms and devices it's No longer a question of which platform first - Android or IOS, it will also include Wearables like Google Glass, SmartWatches, IoT's, VR (ie Occulas), etc so plan ahead.