Optional characters a dated design choice? How do you mean?
It's outdated in that to do so now wouldn't be the reason they were optional before; Characters that the developers really really liked but just couldn't find the time to work fully into the game so half-measures had to be made. Given the opportunity, they might still have been optional but the ability to incorporate them more fully into the game is also there.
At this point SE is relying less and less on prerecorded CGI footage to tell their stories. Games like Kingdom Hearts pretty much didn't use FMVs at all except as eye candy for the introduction and ending of each game. Games like FFXIII-2 have even started incorporating fully real-time sequences where the exact same events can happen but with alternate costumes.
In the era of the PS4 and soon PS5, it's likely going to be possible to have entirely different sequences of events play out in real time based on different event flags, and storage space and the decrease in required file-size to portray prerecorded high quality CGI means that even alternative CGs with different character is a potential option should they feel the need to go that route.
With that said, there's nothing to say they need to be manditory, except a presumption that any hard work put into those optional characters could possibly not be acknowledged if a player (somehow) decided not to pursue those characters. I can't imagine why anyone would skip out on two of the more popular B-roll playable characters, but there you go.
