I think it's more correct to say that both games do not focus on the same things. WoW famously pushes their high-end pve content as a major feature while FFXIV pushes their story content as a major feature. Both are valid. Neither type of content is better or worse than the other, it's about what a player personally prefers. There are players who enjoy both types of focuses and do play both games, they're just attracted to them for different reasons. Variety is nice to have. (and I'm quite disappointed that I still don't enjoy the direction WoW is going in because I'd love to have another mmo I enjoy to play on the side, I was looking forward to New World launching but that has been postponed D': )
I think the ai would have to be more sophisticated for trials for the simple fact that trials typically have mechanics that are more complex than dungeons and mistakes are less forgiving. Dunno about you but I have seen Trust npcs mess up mechanics by standing in telegraphs due to pathing issues. I think they would have to fix that first before expanding Trusts to bigger parties than 4 man.
And like I said there are significantly more 4 man msq instances than msq trials, so just for covering a lot of content it would make sense for SE to prioritise 4 man content first.
I actually quite enjoy Trusts when I want to do a dungeon but I'm in a super chill mood or I'm just curious to see what little bits of lore are hidden in them. I just wish the required xp gains on the npcs to lvl up wasn't so ludicrously high. But I suppose it might be like that because SE don't want players to rely on Trusts too much? It is possible that this is the intention considering they still haven't addressed that and it's the end of the expansion now. But players only need to complete msq content once to progress so it's not a huge issue and they're always available when you have a newly unlocked msq dungeon, and there are other avenues for xp when lvling up other classes.