iphones and imessages are incredibly easy to automate for spam. Today's free, grey and dark markets are often powered by real devices as it's easier to maintain than to reverse engineer and replicate. Usually real devices are mixed with automation code for best results (e.g. real devices generate fingerprints and sign stuff while code automates actions).

Creating barrier to entry for spam definitely reduces it but we know for a fact that's not a very effective spam fighting strategy.

I also think modern anti-spam tech is really good. My Samsung phone here is really good at blocking robo calls here in Thailand. In fact I handn't received one since my upgrade to S22. If Samsung can block robo calls and open protocols like federation and email can stop spam through simple tech and volunteer work then multibillion company with some of the best engineers surely can't find this that challenging right? So I find the spam argument for closing off iMessages not very convincing to say the least.