That totally is not a fair comparison. You buy different phone brands, you still get access to the same set of universal features (roads) like phone calls, texting and the added bonus of an open App Store with potentially infinite optional cross-platform solutions.
iMessage is and was never advertised as a cross-platform solution. It‘s more of a private, gated road you need access to (just like WhatsApp is, you need to register with their service: only difference is, WhatsApp lets the vast majority of devices in, whereas Apple only wants theirs to be let in).