In effect, you are using a third party app (as opposed to stock SMS), but Apple has integrated it into their OS and refused to release versions for other platforms.
https://wabetainfo.com/whatsapp-beta-for-ios-23-19-1-71-what...
I don't want multiple ecosystems that don't interoperate at all. I don't want to give my private conversations over to an ad company like Facebook. I like iMessage because it adds features without taking. I don't worry about what app I need to use depending on who I want to talk to, I just send them a message and move on with my life.
You can run iMessage on Apple devices, nowhere else.
Surely you see the difference.
I know what argument you're trying to make, but I think it is disingenuous to pretend that WhatsApp is free while iMessage is not. As a practical matter neither is free, and both are free.