Will never happen, for a lot of reasons.
The biggest one is that removing the limit on assault means removing the limit on nyzul isle. Both of these are heavily tied into obtaining mythic weapons. Mythic weapons now have magian trials to increase their dmg or enhance the stats on them. For many jobs they will still be some of the best weapons in the game. Never in a million years will SE make getting mythic weapons easier.
Another reason is that it's not out of the question to think the nyzul gear sets will be upgradable from magian trials in the next update.
Yet another reason is assault is also tied into salvage, and again salvage gear will probably have magian upgrades. Also the only way to get alexandrite for mythic upgrading is from salvage.
Also the extra assault tag from completing the ToAU missions would be useless, giving people even less incentive to even bother with the missions.
Best I would think you can hope for is for SE to remove the 3 player minimum on assaults so some of them can be done solo.