Rather than unskippable cutscenes I feel they simply could have that blue ring wall from the beginning of duties that prevents people from moving forward be implemented for final bosses in duties too until the first time new player has finished their CS. That way old players are not forced to visually watch it like MSQ roulette (can do something or chat while waiting) but new players can still enjoy. Also if everyone in the duty has already seen it then the game can just not implement that. A neat lil' flexible system based on party.

We at least know the game can track who has done something or not so doesn't seem too farfetched.