I think I have some unique perspective here, actually, as an American who's been living in the UK for the past year.
I think you're right about the feature set. It's a big historical win for iMessage, and it took a while for Telegram, WhatsApp, etc. to catch up.
I think the other thing here is that a lot of people in Europe have friends from other countries, or travel outside of the country often, and European phone plans don't generally offer good international rates. In the US, most plans offer unlimited SMS/MMS, including international messages -- it wouldn't cost me any more to send SMS to people in Europe from my old American plan (i.e., it would still cost me nothing beyond my base plan price). For Europeans, using WhatsApp is a convenient runaround for this issue. It's a single platform that works internationally, with no particular hurdles aside from "Do you have WhatsApp?"
As an extra little anecdote about the challenges present here, I had to get a Google Voice account and transfer my American number to it, and I now use the Google Voice app to message my family back home. It's pretty annoying, and generally a terrible experience for me, but getting people in the US onto another messaging platform is pretty much a lost cause at this point. I have all of the major messaging apps on my phone - iMessage, WhatsApp, Signal, Telegram, Facebook Messenger, etc. But I just text with most of my American friends, except some of the more privacy-conscious tech nerds who use Signal.