Standards come out of standards bodies. There's nothing other than a law or a lawsuit or the threat thereof that would compel to open iMessage as a standard. It's a competitive value-add to adopting the platform.
Free markets are I create something awesome and profit from it not am then forced to allow you and everyone else to build off of my hard work unless I choose to or am forced to.
If Apple was approached by governments or other institutions to use iMessage as the basis of some standard which iirc hasn't been done that'd be one thing. What we do know is that RCS is inferior to iMessage in many ways.
So Apple will adopt standards compliant RCS but it likely won't be the same and lazy Android users who want to stick to the stock app will get to enjoy the spoils of a half-baked solution.
My issue is with this mob mentality of forcing the hand by fiat, lawsuit, or otherwise to coerce a company to open up a tech/product/platform when they built it with no intentions of opening it -- it's proprietary by design.
The free-market works when someone else builds something -- in this case a better phone experience -- that would cause folks to move to that and abandon iMessage. But no -- let's keep our pitchforks and keep insisting some wierd collective ownership bias that means you or I or anyone else has some say over the property and tech of another person or company. Because, sure that makes sense.