Some of the problems with iMessage have to do with the fact that it's integrated with the system SMS app. It seems that there are a large number of legacy requirements in the GSM spec that require the Messages app to be privileged in some way, especially with regards to automatic processing of data received. There have been plenty of iMessage or Messages related vulnerabilities.

I do wish there was a way to turn off automatic downloading of attachments like images etc. from at least non contacts. I think many/most other chat applications, by default, sanitize and/or format images and media on upload on the server by transcoding image data to prevent things like this and save bandwidth (e.g. Facebook/Meta appears to recompress images server side). However, there are obviously security and privacy considerations to doing this, and the client is not exactly something you want to trust to do this, so I can understand why they would be reluctant to implement something like this.

Perhaps this is one potential use of Treacherous (trusted) computing/remote attestation - the client runs a remotely attested signed binary code that will read an image file, encode the pixels as a jpg, and output a signed output that will only be accepted by the server if untampered.

Obviously, there would be issues with that approach as well, but it could potentially prevent the use of the iMessage network to send "crafted" media files.