Yes, you have described the problem perfectly. If carriers supported it (which they have no interest in doing so and would rather Google handle the larger data sizes of RCS on THEIR network), just like SMS/MMS, then the messages would get to an iPhone and Apple would have to deal with it. Before the iPhone supported MMS, those messages would get to the phone, the phone just couldn’t display them. But, because the carrier supported MMS, that meant that Apple phones would be disadvantaged without support for that carrier feature, so it was added.
No carrier support, no feature.