People need to remember one important thing: FO4 is the first game from BGS that is intended solely for Blu-Ray release.
All prior BGS games were intended for either DVD only or DVD plus Blu-Ra for the PS3.
In addition, Bethesda (for some ungodly reason) has always decided to restrict their game development to a single disc no matter what and despite the fact that (1) multiple disc games have proven to be successful for other companies and (2) BGS games would certainly benefit from not being restricted by disc count due to the sheer volume of content of the games (i.e., BGS has had to edit/remove/reduce content and/or quality every time, pretty much, due to this restriction).
I recall that Todd stated in one interview after Oblivion's release that the voicing of every line of dialogue wound up consuming about half of the space on the single DVD. It was their first time doing this and it was done due to complaints by vocal (minority, IMO) of fans about Morrowind lacking this "feature". Obviously, this required other content to be reduced/edited/restricted in various ways to maintain a single disc release. The same thing has happened with each game since then.
They have far more freedom now with FO4 and only Blu-Ray space limits, so let's see how it works out.
BTW, a voiced protagonist (or not) has no impact on a game being open world RPG or not. There are many examples of both throughout the history of gaming. It's the same as skills not mattering for whether or not a game is an RPG (i.e., RPGs do not need skills and many RPGs do not use them but are still excellent RPGs). Again, this is a first for BGS, so they are going to learn and move forward from it.