Earlier quoted context omitted.
That doesn't seem hard, people like your father can be allow-listed, he's not signing-up out of the blue. (I'm wondering if this "works" because the early adopters were hackintosh types who were already on imessage and not abusers. When the H3rb8l V18gr8 crowd shows up, that is the end.)
Now then.. if I am a professional spammer (which I am not), what stops me from buying a second hand mac or a second hand iphone (for $150 each) and start 'doing business' with iMessage? Unless it's an issue of 'iPhones can't be JB-ed like Androids can be rooted to run all shorts of malware/spamming softwares'. I get Apple's "this is our toy, you won't be making $2/month/user on us". But "keeping the spammers out" it'…
We're also talking about old, non-updated iOS versions here, jailbreaks are more likely to last when they target these versions.