The biggest challenge for android IMO is Apple's Messages. It's clearly a degraded experience having an android user in the GC that some people will create a second GC without the android user in it.
For whatever reason WhatsApp, Signal, Telegram, and others never won out on Android in the US so the Android users didn't bring along their iPhone using network to those clients either (in bulk).
Google of course has lost out because of their own foolishness: https://arstechnica.com/gadgets/2021/08/a-decade-and-a-half-...
When the smartphone era began cellular carriers required customers to buy both data and SMS/MMS components of their service plan. iMessage let iPhone customers save on the SMS/MMS tariff portion of their cellphone plan. [1] Eventually carriers reacted to protect their revenue by simply bundling unlimited SMS/MMS and calling into a new higher base plan price. That killed motivation for Americans to adopt a free alternative like Whatsapp unlike the rest of the world. The messages app on iOS mostly abstracts away whether a message is delivered over the Internet or legacy MMS, except for this blue/green color.
Apple considers iMessage popularity an important vendor lock in moat. Carriers don't have any motivation to deploy RCS as it won't yield any more revenue, and especially since Apple won't. Customers strongly refer free messaging over paid and take the path of least resistance. People generally just pick the lowest common denominator communication channel instead of burning time and social capital getting their friends and acquaintances to install Signal, Whatsapp, etc.
Message fragmentation is also not helped by Google's constant re-branding and product cancellations. In markets where non-smartphone owners cannot be ignored, the lowest common denominator is still SMS or email.
[1] https://9to5mac.com/2012/05/04/att-ceo-imessage-disrupts-our...