We sort of have that in the form of FATEs...

I dont think it would be a "bad" thing, it would be a "different" thing - there are pros and cons to both ways and at least for FFXIV the devs simply choose the way of fixed partys now.
To mention a few pros here:

1) It allows for more interesting mechanics since the team knows excatly what party-composition they're dealing with
2) Looking at large FATEs or hunts this game obviously cant handle large amounts of people in one area, fighting one mob
3) Screenclutter - at least for me 24 people in the 24-raids are enough already to fill my screen. This might be a personal preference, but if your personal preference is to have more characters on the screen there seem to be options for this and you can choose one of them. Not every game has to appeal to you 100% or has to change to achieve that.

and a question: do you mean it will scale at the start or in progress?

I think you fail to see that those are two types of games appealing to different playstyles - one is a lot more focused on just the gameplay (you play battlefield because you want to play that massive shooter), the other one has a lot more to offer. Dungeons are almost always tied to the story, so having them cluttered with a lot of people doesnt really make sense.
In the end its a design choice...