The fundamental issue is that there are no readily-available iPhone emulators that can send iMessages, and this is a good thing that people pay for.
The benefit of the Apple walled garden is that the spammers are kept out, because they can't get fifty SIM cards, plug them in to a "SIM box"[1], and start sending hundreds of thousands of scam messages via iMessage. This is easy in an uncontrolled, open-source ecosystem, but is prohibitively difficult in Apple's ecosystem where you need a physical device with a unique cryptographic ID that can be locked out by a central authority.
Yes, it's an "exclusive club" where members pay to keep the general public out. The general public doesn't like this, but that is what it is. Inherently, allowing "anyone" in would void the exclusivity and invite an unrelenting flood of spam.
At best, Apple could open up iMessage to other vendors that have similarly well-controlled devices with good hardware cryptographic attestation, but there aren't any.
This argument doesn't make any sense from a user standpoint though. It would only make sense if users insisted on only using iMessage exclusively and didn't accept non-iMessage SMS messages at all. Getting an iphone and using iMessage doesn't make you less susceptible to spam text messages than Android users or people using other texting platforms, *because everyone uses SMS*. It's impossible not to.
Isn't there? You can use iMessage via the Messages app on a Mac and tools like BlueBubbles already have hooks to send and receive iMessages using the official client.
How so? You can easily automate sending messages on a Mac, and you can easily change the IDs on Intel based Macs. Ultimately, the only solid line of defence is banning your phone number and Apple ID.
The benefit of the Apple walled garden is that the spammers are kept out, because they can't get fifty SIM cards, plug them in to a "SIM box"[1], and start sending hundreds of thousands of scam messages via iMessage. This is easy in an uncontrolled, open-source ecosystem, but is prohibitively difficult in Apple's ecosystem where you need a physical device with a unique cryptographic ID that can be locked out by a central authority.
Yes, it's an "exclusive club" where members pay to keep the general public out. The general public doesn't like this, but that is what it is. Inherently, allowing "anyone" in would void the exclusivity and invite an unrelenting flood of spam.
At best, Apple could open up iMessage to other vendors that have similarly well-controlled devices with good hardware cryptographic attestation, but there aren't any.
[1] https://www.smh.com.au/national/nsw/sydney-man-charged-with-...