Okay, sure, that’s just all of Apple’s software though. With a few exceptions (Apple Music and Safari for Windows (RIP) I believe) all their software has only been available for their own operating systems. And (especially since I’m a Linux user) it would be great if cross-platform software were the standard, I don’t think software can be truly cross-platform without being open source. And as much as I think forcing every company to open source all their code would be epic, I don’t think it’s reasonable, as much as I don’t think it’s reasonable to force them to port to every platform.
Rather I think that generally all software anyone can legally obtain should require any sort of file format, network protocol, or other protocols that are used to transfer information between computers to have (usable) public domain documentation, plus, in case the protocol makes use of device authentication, anyone to obtain a valid certificate for their device. This would solve the iMessage problem because it would allow anyone to write clients for it for any platform, but it would solve the same problem for iCloud, Microsoft Office (LibreOffice could have so much better compatibility if they didn’t have to reverse-engineer the file format), Photoshop, Dis “custom clients are against TOS” cord, and thousands of other proprietary software. Because those are all the same exact problem as far as I’m concerned.
Maybe you’re saying something along the same lines. But I don’t think it’s specifically an iMessage problem.