How would solo be any less limiting, if not more so, than the current Trust system that exists?
Unless they gutted the damage entirely, how would you be able to survive stack mechanics? If they want to add the multiple towers that requires one person to stand in, how do you resolve it?

They can't, unless you just face tank those mechanics in which case helps no one, at least with Trusts it provides players a way to get a feel for the mechanics before jumping into duty finder.

This doesn't even include the extra lore bits they include for Trusts with the NPC dialogue, for those that enjoy that.