Protocols calcify when you don't control all the endpoints, consider the case in point, iMessage, it is seems like there is some security implications for spoofing iMessage for any random number, yet, apple's recourse is very limited if it can't update all the endpoints (devices).

The same is also true, say about AirDrop, if apple makes it "Open" and they have to make a breaking change for security or whatever reason, they can't feasibly even make an update available for non-apple devices let alone enforce it.

Now "Apple" has broken your non-apple device and along with it their reputation.

Open is good, but the cost is non-zero.

By that logic the Outlook for Windows team should be responsible for patching Gmail for Android.

This argument is silly. You could use this line of reasoning to justify why all computers should use the same OS from the same vendor. Of course then you'd have a monoculture where implementation bugs that cause vulnerabilities are universally exploitable, instead of only exploitable on machines running that vendor's software.

This isn’t uncommon at all when dealing with development that requires interoperability.

Far from it, actually.

If a part of your user base uses another service, you’ll inevitably have to add workarounds specifically to cater to users for that service. It’s just a fact of life when multiple groups have to implement a spec. If you aren’t willing to add workarounds, users will think your software is broken when they should be blaming someone else.

For example, Firefox maintains a few workarounds for websites that ship in the browser. They aren’t the web developers responsible for the sites but someone has to make it work.

Interoperability is not free.

Not free, but worth it for messaging to solve the real pain points that iMessage and Google Messages users deal with while trying to communicate across the aisle.

You're free to make value judgments, but for a business to follow through, the economics of it must be sensible. As a consumer, I prefer a secure messaging platform over an open one any day of the week.