Honestly, kinda figured/assumed/expected it would be 100% new trust members going into 6.0. Otherwise, new players just coming through the story will, by the end of 6.0, have old characters still at 71 to level to 90, as well as new characters at 81 capable of running any/all Shadowbringers content without needing to level them. Plus, for everyone that's already leveled trusts to 80, there is the matter of the awkward state of needing to thus level them up to 81 before you could take any current characters into 6.0 content. And who knows how/if current trust members would adapt skills/rotations going forward. Would be a bit lame if the AI doesn't change at all.

Overall, it just seemed like the best/easiest way to maintain the system as is, as designed, would likely be having a whole new group. One per expansion. Allows them to avoid any troublesome quirks of cross-expansion Trust runs that will require additional foresight.