This is without a doubt an incredibly divisive issue. There are really only two camps here. Those in favor of the voiced protagonist and those against. I'm not really interested in the political argument of why more freedom for RP is better or how the voiced performance provides for a more compelling and interesting story. What I am addressing is why this is here to stay and likely we will see a voiced protagonist in TES VI (although due to the nature of ten races that could be problematic).
1. All the other major open world/RPG competitors are doing it.
Voiced protagonists are nothing new to games. However, a voiced protagonist with varying choices to choose from is a relatively new invention. Most would largely credit this to BioWare with the creation of the dialogue wheel and the voiced protagonist of Shepard in the Mass Effect trilogy. We have seen it replicated in their other property, Dragon Age, as well as CDPR using a similar system in The Witcher 2 and The Witcher 3. Not to mention, games even without a dialogue wheel of choices provide a voiced protagonist in virtually every medium. Whether it's Grand Theft Auto, Bioshock, Assassin's Creed, Call of Duty, Uncharted, The Last of Us, Tomb Raider, Halo, etc. A voiced protagonist is without a doubt a component of AAA gaming today.
2. BGS storytelling and main quest lines have been historically weak.
BGS has never been known for making a truly compelling main story experience. Going as far back as Morrowind, BGS has never had a main story for any of their games really stand out. In Morrowind, you are Nerevar reincarnated to defeat Dagoth Ur once and for all. In Oblivion, you are essentially Martin Septim's messenger the entire game trying to prevent Mehrunes Dagon from invading Tamriel from his plane of Oblivion. In Fallout 3, you are a vault dweller looking for his father and ultimately uncovering the secret to water purification. In Skyrim, you are the prophesized last Dragonborn who is destined to battle with Alduin, the World Eater. Even though I believe BGS made a decent effort with Skyrim, none of the stories ultimately really rivaled other competitors in the industry. Whether people agree or not, this was in part because BGS games have been one of the few in the industry to maintain the silent protagonist approach.
3. BGS is looking for ways of innovating upon their winning model.
BGS is a pioneer. They are always looking at doing these they've never done before in their previous games. In Oblivion, it was features such as Oblivion gates and Radiant AI. In Fallout 3, it was VATS and moral choices. In Skyrim, it was dragon fights and Radiant Story. With Fallout 4, it's a voiced protagonist, the overhaul of crafting, and settlements. BGS is always trying to innovate and never shy away from trying new things, especially in areas they underperform. Storytelling has always been a major thorn in BGS' side. It's the one thing nobody has largely liked about any of their games. It's also one thing a game should not ignore because story does matter. Especially in a role playing game, the entire point of role playing is to create a great story, and that has just never existed in BGS games. They were always better simulators than actual RPGs. Thus, making this leap and trying to tell a more personal story just made sense and was a way in which BGS believed they could improve the storytelling experience.
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Why is a voiced protagonist unlikely to disappear from BGS games? For one, it's their first attempt at it and they are largely getting a feel for the process. They had to determine how to allocate resources to a voiced protagonist and how to craft the dialogue in a way that can provide for a variety of varying responses. Secondly, BGS isn't known to just add a game changing feature and then take it out subsequently in the next title. They innovate upon it, tweak it, improve it, and figure out ways of making it better.
Again, it begs the question how exactly this would work in TES VI. With ten races, it would be inappropriate to have all of the races sound the same for males and females. Of course, BGS could restrict what races players can choose in TES VI, but that has never been the tradition in any of the titles going back to Arena. We'll see exactly what BGS does. Perhaps this will be merely a Fallout innovation, much like VATS, but I doubt it. I believe a voiced protagonist is here to stay and I believe BGS is right that a majority of gamers will be content and probably like the addition of a voice. Obviously not everybody will be happy, but that is the case with any feature in any game. It's a matter of a cost-benefit anolysis and determining if X feature will lead to a better Y result.
I have no idea what the numbers are for Fallout 4 on consoles, but without a doubt Fallout 4 was breaking numerous records on Steam. For one, it had over 450,000 players logged into the game at one time, which is a record that not even GTA V could beat. The success of the game alone will likely justify that BGS made the right move. In other words, with how well the game is performing, I find it unlikely a voiced protagonist will not return for TES VI and eventually Fallout 5. BGS doesn't add a new innovation to their toolset if they aren't fully committed to the cause. I don't see a voiced protagonist being any different.
I know for some this might be a deal breaker for you, but the same exact discussions happened on BioWare Social Network with regard to the Mass Effect dialogue system and voiced protagoinst being implemented into Dragon Age. People to this very day will continue to attack BioWare and tell them that a voiced protagonist was a mistake, yet the Mass Effect trilogy is BioWare's most profitable original IP and it only made sense that Dragon Age would follow suit. BioWare even applied the voiced protagonist and dialogue system to SWTOR, their first MMO, so regardless of how one may view a voiced protagonist, money seems to suggest it leads to better results.