Until RCS is supported by all carriers and device manufacturers (or messaging app developers), cross-platform media sharing WILL be a problem.
I don't think that's in carriers best interest, unless there's a guarantee that consumers that switch from iPhones, will switch to an expensive Android flagship.
If there is a lack of need for iMessage, then there will be an influx of consumers that will switch to Android. Carriers don't want to risk switchers choosing an Android device that is hundreds of dollars cheaper. That will affect the money they make on financing and in-store accessories that mostly cater to flagship models.
I wouldn't be surprised if Apple already had that conversation with carriers.