TL;DR: Apple dominates the US smartphone market, but EU regulations may offer Android a chance for resurgence by enforcing messaging interoperability and standardizing hardware features.
It's exactly the same argument as with Windows and Internet Explorer
No, it's not. Maybe I've looked at the wrong numbers but according to https://www.counterpointresearch.com/us-market-smartphone-share/ iPhones have a market share of 50–60% in the US, not 90% like Windows on PCs. It shouldn't result in total iMessage dominance. If anything the somewhat equal market share should mean that Telegram/Signal/WhatApp/whatever should be especially popular.