Repeating something they did from DO2 is not "design philosophy" its them turning themselves into a one-trick pony. Worst case scenario they felt they did the idea wrong in Do2, and felt the poor response was due to mishandling and are saying to themselves "we will do it right this time"
Instead of coming to the more obvious conclusion "this is a thing most games dont do... for a good reason"
And in this game, a game about a group of disparate people who are from vastly different backgrounds and existences, who under different circumstances would NEVER be friends... having to come together, and learn to work together to solve the same problem and situation they ave all been thrust into... is extremely undermined if the party is broken up for any reason.
Breaking the party up after Act 1, and forcing the player to commit to one party... is not mechanically advisable, it is not narratively advisable, it is not good for the image of there company... it is a bad idea.
You can argue terminology all day long. The point is that the reason they are doing it has nothing to do with the story they want to tell.
I think it's a shity choice as well. What I tried to add to this discussion is the fact that I still don't understand their reasoning. If they were to give a better explanation other than the "you have to commit" bullshit I might have considered their view, but since they haven't, I'm on your side.
I did have a concept for a game that companion locking would work on, but it's a very different structure basically:
Act 1 - there's a collection of quests and side-quests available which leads toward building your party... but quests don't really hang around and just wait for you to take them...
So, off quest A, B, C, and D take quest A and leave area to head to do that quest do it then return to hub and now there's quests C-2, E, F, and G to take. By end of Act 1 you're pulled into a longer-scale quest that's basically you and the companions you already recruited (maybe enough to cycle some out for different side-quests) that goes to Act 2 which leads to Act 3.
Then on replay you can choose an option to set it in same time-line as prior games and run a different set of quests dealing with other problems and maybe encountering tale of your older party now and again.
I'm sure you can do it with good writers, the question is why do it? Is it important to the story you wish to tell or is it just something some higher up in the company decided and the writers have to follow in line?
It's a question of the chicken and the egg. If you do it because this is how you envisioned your story it could work and work well. But if you do it only because of some arbitrary bullshit decision and you build your story around it, the chances of the story working are getting very low.