iMessage is a huge factor in keeping some people on iOS. When I had my little "bored of iOS" episode and tried a Nexus 6P and S7 Edge, I found that the worst possible messaging experience is on Android.
All of my buddies have iPhones, my wife and kids have iPhones, most of my co-workers have iPhones and the Group MMS thing on Android is abyssmal. I shouldn't have to ask the people I communicate with to switch to a different messaging service just to appease me....
There is a known bug that because I switched from iOS to Android, when I would start a new Group MMS to my friends, it defaulted on THEIR phones (which was started on iMessage) to iMessage and none of my new texts to the ongoing conversation would work. No one got anything. Only fix is to have everyone delete and restart conversations....My buddies HATE having to delete their messages. So it's real a pain.
A universal app would help a lot of people who use Android for work phones, or if something happens and they have to switch from iOS (losing their iPhone and having a temporary phone, etc)