It's not fanservice to include a previous character in a role that suits them.
If they were included in a role that wasn't suitable for them, or turned up with no role to play in the game it would be fanservice. Bioware could have invented a new character to play these roles, but there was no reason to when they already had people that fit. I don't even like Leiliana or Cullen or Alistair, but they weren't simply fanservice.
Yes i somehow agree(even if others people could have been used at their place) they are necessary and well suited in their role, they are not in DAI for the sake of fan service,they are necessary and as a personal taste i prefer Cullen and Leliana rather than some new x character of whom i do not even care about since they are perfect for those roles.
Alistair and Loghain are however not necessary because Stroud is always alive in DA2 and imho it does not make sense to have 3 charcaters to do the same thing and put them there just so that they could have another chance to die(Alistair/Loghain)
Morrigan on the other hand,with or without her it makes no difference since it is Flemeth the one which is vital to kill Corypheus and for the temple of Mythal they could have simply locked Solas as mandatory
Yes it supposed because she was necessary to finish/carry conflict devs started in first game and writers set on future product.
Necessary to finish what?
The OGB plotline which is not mandatory?
If the dark ritual was not performed and the well of sorrow was used by the Inquisitior FLemeth has no reason or necessity to want to meet Morrigan.