It's a solved problem as in it's one single problem that is solved. I agree that WhatsApp is a really bad solution overall, just compared to iMessage it does solve the cross-platform issue.
I would also by far prefer a more open solution, but between relying on Apple for your country's messaging to relying on Facebook, at least by relying on Facebook you have one less issue.
I would also by far prefer a more open solution, but between relying on Apple for your country's messaging to relying on Facebook, at least by relying on Facebook you have one less issue.