The fact that SMS group messages suck isn't something that Apple can really fix without an oob protocol which would basically be iMessage again.
It tough to blame Apple for not open-sourcing a protocol they are sued about by a patent troll. Until resolved properly this would be a toxic code-base anyway.
Of course they could restart from scratch, but I guess they have no incentive to make efforts about that. What will be meaningful however is be what they'll do after patent exhaustion date of infringed VirnetX "patents".
[1]: https://www.reddit.com/r/apple/comments/1xuzif/what_ever_hap... [2]: https://www.engadget.com/2017/10/16/apple-faces-440-million-...