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I'm curious about this one.
I agree the content is lackluster - though probably only can be felt by veteran like us.
But what open world game that doesn't have this content issue?
Witcher 3, current game of the year has this exact issue, beautiful world but boring sidequests. Not sure why you said CDPR flawless function/experience wise (it has 600+ bugs at the beginning and combat doesn't fixed months later). Witcher 2 on the other side is almost Telltale game.
GTA series, the billion dollar franchise has the same issue (go there, kill that - even on main quest). You can do plenty of other things but mostly silly or one time amusement things.
Far Cry series, the same. Worse, the "liberate outpost" thingie is the same between AC, FarCry, and WatchDogs.
Even highly praised games like Bioshock, Wolfenstein, Dark Souls 2, or Tomb Raider eventually feel repetitive at 20-30% completion, and that's about 10-30 hours in the game.
Just Cause has virtually no story, and doesn't pretend to. They invested time into producing the best possible murder sandbox, and gave you enough tools to do it however you see fit. The saving grace for this series is it doesn't pretend to be a story, only a murderous sand box.
Far Cry 3 and 4 both had immensely like able and animate villains. Their random encounters were actually pretty awesome, and I hit the end of the game well before the content reached the point of being boring. The saving grace for this series is that the game is paced such that content nuggets exist well into the story's end, and they don't exactly pile in a ton of frustratingly samey side missions.
Terraria is the definition of a sandbox without a story. The developer has absolutely no story in the game, no objectives, and yet I've poured hundreds of hours into that game. Between mild modding support, and the ability to set your own objectives without interference from half-baked mechanics, Terraria is interesting because it just does what I want it to do.
Let's now focus on Bethesda's progeny, to show why the above doesn't apply.
First off, there's a definite story with multiple players. That means you've got to pace content such that its consumption and your mechanics are not at odds. By linking level progression, perks, a level curve that starts to overhang, and constantly repeating quests as the "reward" for a faction you wind-up at odds here. Removing the level cap is great, but in its place you've installed a system which requires players grind to get fun perks. This, in and of itself, isn't a bad thing, as it allows casual players to progress, while hard core players can chew through content as they see fit. The problem then becomes whether or not there's content enough to cover your infinite progress scheme. Bethesda doesn't have this. As you increase in level the enemies get better stats constantly, but you run out of weapons to match their additional statistics at about level 50. Really, the plasma cartridges are surprisingly rare and that's arguably the best weapon for most players (explosives being of limited use inside areas, and melee becoming difficult at high levels). As you continue to level there's no way to make yourself match enemy progress, because perks run out and weaponry does too. At this point enemies become bullet sponges, and the grind becomes apparent to the player.
Second, is the game 95% mechanically. Absolutely not. CD Projekt Red did eventually fix almost every error in the Witcher 3. Their errors were therefore annoying, but tolerable. Let's look back at Bethesda's work. Fallout 3 is, to this day, an unstable mess without community mods. Likewise, New Vegas (a project they had oversight on, but did not develop) is still buggy today. More recently, let's look at Skyrim. Bethesda put out patches at lightning speed on that game. Unfortunately, each patch seemed to break something new. My experience with that game was that I fought a dragon without any issues, had it respawn and it died without issues a second time, and two patches later that dragon flew backwards circles around me without any animations before teleporting off. I can accept errors, but Bethesda has a track record of either causing more during "fixing" or entirely not addressing them. This breaks the game, because every CTD puts me 2-3 minutes farther away from enjoying the game and its excessive loading screens.
Finally, can I just explore? Surprisingly, the answer there is no. Often a radiant quest involves you going somewhere and killing a named NPC. If you explore constantly you murder these enemies as a matter of course, and only hours later find the NPC who could have filled you in on why they were special. Worse yet, this is easy to rectify. If Bethesda could be bothered spawns would be related to quests. In reality, they aren't. You can fail missions by killing hostiles you encountered at random, kill named enemies without ever knowing why they're named, and all of this just because exploration is definitely not the focus of the game. Bethesda has done a wonderful job giving us one complete story, and hundreds of little ones, but they've done nothing to link them together. That's just depressing, when so much effort in developing the world is lost simply because we didn't play how they wanted. We didn't decide to cut off viable content in order to finish the main story. That's a disappointing lack of thought, that we've tolerated from Bethesda for years. I'm beyond even asking them to fix this, but not raising it as an issue lets them say it's acceptable. I'm definitely not accepting it like a sheep.
This game isn't bad, but with seven years between installments on the same engine I expected better. We should ask for better.