WhatsApp gained popularity in Europe when non-early adopters started switching to smartphones (2010-2012 ish). The difference was that, in contrast to iMessage, you could install it on pretty much any smartphone. In the pre-Meta days they even supported many then dying ecosystems like Symbian, Nokia S40, etc.

They had a yearly subscription fee, but most people never got the request to pay it.

Nobody cared that it was incompatible with SMS, because everybody hated SMS because of the insane prices. In 2009 I got an iPhone 3G with an unlimited data plan, but I was still paying something like 0.20 or 0.25 Euro per SMS.