iMessage remains an also-ran in the messaging world. Most non-US countries use an alternative like WeChat, WhatsApp or Telegram. Teenagers don’t ask romantic interests for phone numbers but Snapchat handles instead which makes managing and blocking people a lot easier.
Proprietary apps that ignore a majority of users are yesterdays news. It wouldn’t even matter is Apple released iMessage on Android or adopted RMS. People are already engrained in their cross-platform app of choice.
I say all this but I do actually quite like iMessage as a platform. But iMessage isn’t a selling point anymore and indeed Apple need to pull their finger out sooner rather than later because their current coasting is making that list of selling points shorter every year.