I would go with instances being solo, with NPC companions (Scions) and last bosses turned into Trials (Livia and Ultima respectively).

Remember that for new players, Merridianum is one of first 8-man dungeons, and its usually the first place where they realise that in 8-man, other players are NOT locked until everybody finishes cutscenes.
Therefore you finish your first cutscene and see the whole party is gone, probably already fighting first miniboss.

As a new player, I can confirm the current experience from game immersion perspective is abysmal for both these instances.
It is by far the most story-breaking experience I have seen on my way to 70, second being the super boring filler-quests plaguing questline preceding Heavensward.
The HW and Stormblood MSQ are luckily huge improvements.