You don't. You get no-fee access to iMessage with your Apple devices (phones, tablets, laptops, desktops). Why is that relevant though? Why should Apple be forced to support apps on their competitor's platform?
Suppose Apple had plans to monetise iMessage at the time it was developed and eventually decided against it for whatever reason. This would leave the company with a an app running on most phones. What to do? I can easily imaging someone standing up in a meeting and suggesting: "Well, running it costs very little, why don't we just minimise the cost of running it and keep providing it for as long as the cost is low?"
That doesn't make sense to me. It makes monetisation be a synonym of funding.
Surely, monetisation means that you do x in order to earn income from hardware sales. That is to say, your motivation counts, or rationale. The rationale is of course only known to people who attend the relevant budget meetings.
In America, iMessage got there first. Even though it's technically worse than WhatsApp, enough people use it (a majority) that it doesn't matter that it handicaps its own users with SMS and green bubbles. People think it's all the other phones' fault rather than the iPhone's fault. It's a massive driver of iPhone sales, and Apple execs are on the record acknowledging that.
That's not how it's supposed to work. The better messaging platforms should be competitive. Apple stock would tank if they were forced to open up the iMessage API.
If I were on the board of Apple, that's precisely why I'd be pushing for it to be open. I would be terrified if my company's value were in a stupid chatting protocol. It's the difference between accidentally driving off a cliff and purposely driving off a cliff with a parachute. I'd choose the parachute and make money/goodwill from the exposure vs. losing everything because the "blue sheeple" woke up.
They do this with a lot of things, if you try to use a password manager on iOS other than Apple's, it's so restricted it's almost unusable.
Source: recently switched from iPhone to Pixel, not sure how long I can keep it up though
The argument that iMessage is a service provided by Apple for Apple products and that they should be allowed to do whatever with their products is the same argument they've used to restrict App Store, force developers to use their payment systems and charge outrageous cuts of every payment a user might make through there.
iMessage plays quite a notable role in this, as a user whose friends use iMessage (more prevalent in parts of the U.S. I think) might feel unable to move away in order to be connected to their iMessage-using friends, in turn ensuring that they keep buying iPhones and paying Apple a 30% cut of all their app subscriptions from Duolingo to Disney+.
Apple might be correct in this regard, as iMessage is not very popular in EU specifically. For this to apply here, either Apple would have to be caught lying about their numbers (not impossible), the EU would have to lower the minimums, or the US would have to create their own counterpart as iMessage is much more popular in the US.
It's not even easy to know who's got an iPhone now: it's just a big phone with lots of cameras as any other one. However it's likely that two close friends, families, etc all using iPhones could decide to use iMessage for their own messages. Same as for... (had to google it) FaceTime.