Eh I don't think M+ is in any way a better reward system. The RNG is extreme given how absolutely bloated Dungeon loot tables are. You get a random piece at a higher ilvl from the Vault lottery. The S4 proposed system for raiding IS a better system. Between the Catalyst and the bad luck protection with the three coins you get rewards proportional to the difficulty you complete and have significant built-in bad lack protection. I hope the S4 raiding reward system succeeds and then maybe they can start rethinking M+ gearing which is only really supported by the Valor grind; and even with no Cap it's still a frustrating grind.
M+ is the most played system because it is the least effort for the highest ilvl. That has always been the way this community played. Do you think people suddenly loved Arena in TBC and completely fell out of love for it with Wrath and after? People will do whatever content will get them the best gear for the least effort and that is M+15 right now. Popularity does not equal enjoyment and it is why Blizzard's metrics driven feedback process failed and they are now going back to community interaction.
As for raid+ I don't think that is needed. You just need to do what many other games have done. Add speedrunning leaderboards. It's what kept classic raiding alive for a while and in retail's case, the content cycle would let people keep optimizing. Plus speedrunning might help mediate the log whoring issue with things like PI; guilds will want to maximize raid dps instead of helping an officer get their legendary logs because then the entire raid can get a ranking. You can even end of week rewards or something similar (top X% of logs gets a gift box in their mail)