Apple wouldn't be doing this—at considerable expense—if they didn't consider such attacks to be plausible within the relevant lifetime of currently intercepted and retained messages (ie. not necessarily today, but within the statute of limitations of any crime you might be confessing to in an iMessage).
https://www.theverge.com/2024/2/21/24079081/apple-imessage-pq3-post-quantum-cryptography