I for one am not a fan of this decision to make us commit to a set party. I love interacting with companions and swapping them around throughout a game, and I find the idea of people not swapping characters in and out kind of bizarre personally. If that's fun for you then go for it, but I never could, it would feel like a wasted opportunity.

The fact they're doing this as well as a 4 person party also concerns me since as has been mentioned, that takes away a lot of flexibility. Sure losing out on a rogue and not being able to open chests is arguably a way to add replayability, but only if the game is mindful enough not to put too much quality loot behind locked doors and give ways to avoid traps. But my main concern is for combat. If there are characters whose dynamics I like together but who don't make for a good combat team, then I can't have them together. For instance I love playing magic users, they're my favourite kind of characters to play by far, nothing else even comes close. That automatically means that I can probably only afford to include at best one other magic user, otherwise my party likely won't have a chance in most combat encounters. It precludes certain party combinations without multiclassing, which isn't a great option for every situation anyway, and furthermore multiclassing introduces another layer of complexity which can be a pitfall for players who aren't familiar with the system, like me. If they're going to be leaning on multiclassing as a way to alleviate that kind of problem then they absolutely need to have a solid tutorial that can help players figure it out.

And finally, yes it's true that D&D might be optomized for a 4 character party, but translating that to a video game becomes complicated because unlike in a real life game, your GM can't tailor your experience to the party composition to ensure that an encounter is never accidentally beyond the groups abilities. If you want to have a group of all mages, a GM can adjust the threats you face accordingly. A computer can't do that. So yeah, I'm worried that this particular combination of choices will limit player choice in an un-fun way that doesn't actually benefit the overall experience.