iMessage is just the iOS texting app. When someone says "I'm having trouble getting Stranger Things to play on Netflix" you don't tell them "You should switch to Hulu". Netflix (iMessage / texting apps) has Stranger Things (texting) and Hulu ("alternatives" like Whatsapp et al) do not.
As an Android user, in theory I shouldn't care about iMessage. However, because of the way that iMessage creates schisms, miscommunications, lost communications, broken texting experiences and more between my Android friends and my iPhone friends, I have to. I would like the texting features of these phones to interoperate so we can all text together in peace.
I wrote up a scenario (user story?) that I think helps to explain the problems I think should be solved that seem to fly over so many people's heads, especially when they advocate for over-the-top messaging apps like Whatsapp to solve the problem (particularly in the US context): https://news.ycombinator.com/item?id=38578101