Forcing iMessage to be interoperable with WhatsApp, Facebook Messenger, Telegram, and hundreds of minor texting apps.... will equate to thousands of glitches and bugs every week. Absolutely stupid.
It would be a different story had the EU simply laid forth a strategy saying: "By so-and-so Year, all major texting apps should adopt a single universal open standard, which would easily facilitate inter-operability regardless of what app is being used to send or receive the messages."
THAT kind of strategy would be more adoptable by companies like Apple, Google, Samsung, Microsoft, etc. In fact, they already embrace texting standards to a good degree.... embracing SMS, MMS, and universal Emoji standards.
Simply threatening companies like Microsoft, Apple, or Facebook to "Open up or else" is stupid and near-sighted. Which then results in the chaos and thousands of glitches situation as I stated above.