Ick. I wish we could kill SMS/Whatsapp/Wechat/LINE already and just do everything over e-mail or some other cross-platform, multi-login protocol. I hate being tied up to one device to message people. I change devices on the order of minutes, move over the course of a day between a few locations that all have devices I own, and don't regularly carry or look at my cell phone if I am already staring at something bigger…
Wouldn't RCS deliver exactly what you're asking for?