The issue is for Apple users and Android users alike. If an iPhone user only knows an Android user's mobile number then their only option to communicate is via iMessage and the Google Messages app. Both are default apps. With the RCS protocol on both apps both can send high quality media to each other without downloading another app. WhatsApp, FB Messenger and other common apps still compress media. RCS would fix this for both users.
and that is why apple users have imessage, so the issue isn't an issue for Apple users.