Skyrim was also pretty innovative when it launched, no one had seen a world THAT big and with THAT level of detail before. Cities were large and full of NPC's which went about their daily tasks. Fights were epic and there was no lack of lore to pore over. Say what you will about Skyrim but it definitely set a new high benchmark for open world RPG's and graphical quality.
The fuck are you talking about?
All right, let's start with the innovative part, no one had seen a world THAT big and with THAT level of detail before? Uh... I have? Saints Row? GTA? Fallout 3? Oblivion? Ugh... Morrowind...? The fuck are you talking about, man? There's been plenty of big open world games with a hard-on for attention to detail.
And big cities? What game were you playing? Whiterun has like a dozen houses if even that inside of its gates and like a dozen more outside of its gates. That's a "large city"? Oblivion IIRC had larger cities or at the very least same-sized cities. And NPC's went about their daily tasks? Yeah, so did they in Oblivion and Fallout 3. Hell, Saints Row is a big city and its inhabitants were far more colourful and interesting to follow around.
Fights were epic? I'm... I don't... What? WHAT!? You mean the time an army sent like 2 dozen men to storm a city? That was "epic"? Whaaaaat? What else was epic? Fighting a dragon to defend Whiterun with like half a dozen guards? Yeah... Real epic. Or do you mean the time I got propelled into outer space by a giant? Not really epic. More like funny. What exactly was 'epic' in Skyrim? Cause I can't remember anything I'd consider close to "EPIC!"
As to lore, didn't they butcher a bunch of lore for Skyrim? If they did, then the lore is meaningless then.
And high benchmark for graphical quality? Am I the only one who saw the sluggish and jacky animations? Am I the only one who saw those ugly blocky shadows? Am I the only one who paid attention to the direction water flows and noticed that all water flows into a universal direction regardless of where the river is heading?
I don't get this. When the hell did Skyrim become something to praise?
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Dark Souls, came out in 2011, had way better melee combat.
Mount And Blade Warband, came out in 2010, had way more epic battles.
Two Worlds 2, came out in 2010, had a way better magic system and even allowed you to craft your own spells on the fly.
As to exploration? Well, Oblivion or Morrowind kinda already did this "innovation".
What else is there. Dialogue? FNV came out a year before Skyrim. It played mostly the same and its dialogue system and writing blows Skyrim out of the water.
FNV also had a reputation system that far outmatches Skyrim's "bounty" system it has for its holds.
What else is there? Dragons? Yeah I found fighting a dragon in Dragon Age to be way more engaging and epic.
Skyrim is not innovative and it did absolutely nothing that something else hasn't done before and better.
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Now as to your theory about why Fallout 4 turned out the way it did, I dunno, could be that's what happened. Bethesda is so tightlipped that it'd be hard to difficult to figure it out though. But yeah it seems strange that after 4 years of proper development time this is all we're left with. Then again, I found the design for Skyrim quite lazy and slapper together, so it could just be that Bethesda are bad at making games. I dunno. Like, think of how many enemies there are in Dark Souls and hell, think of how many different weapon types there are for the player to use. Think of all the different animations and models and balancing they had to do. Right? Now think back on just how many different enemy types there were in Skyrim and how many there are in Fallout 4. Shouldn't a triple-A studio have been able to develop more than this? I mean, what do the animators at Bethesda do all day?
So I dunno, man. If it was the engine being a problem for newer consoles then I guess we'll know if they decide to ditch the engine for their next game and go with a different one.