The game really isn't about the main quest, it's like 90% side content, it's more about just exploring the world and finding out cool things about it while having fun adventures and becoming powerful. The plot is just how they kick off the adventure. You can legit play for 200 hours without even touching the main quest.
See, that right there is the problem. The gameplay is the standard "faff about" Bethesda fare. Which is fine on its own, and works as often as not.
Heck, Skyrim had an actual dragon demigod literally trying to usher in the apocalypse, and "faffing about" worked there because you're just kinda thrown into the situation with absolutely no idea of how to actually deal with it. Hell, you don't even know you
can deal with it until a few quests after meeting the Greybeards, and even then you're being constantly given contradictory advice on what you actually should do. The Blades want to use you as a weapon to wipe out the dragons, the Greybeards urge pacifism and letting the chips fall as they may. And in the meantime every hold is falling apart due to a variety of issues, not the least of which is that the Imperials and Stormcloaks are in a goddamn civil war which compromises their ability to deal with the dragon threat (which they dismiss as a nuisance), and the Thalmor are undermining everything.
The game structure works in that case because the story is built around it. The Dragonborn is stumbling around blind with their only direction for most of the plot being a vague "stop Alduin" with no idea of where and how the dragon is trying to accomplish his plans, much less what can actually be done to stop them. Moreover, while there's a vague sense of "it's bad that this is happening", there's no real implication that you have to act
now; at least no more urgent than the typical "dethrone the usurper king" plotline.
So taking your time and making sure that you leave no stone uncovered works...especially when so many of your expeditions seem to be improving your odds by so frequently helping you stumble across words of power. Even if the nominal point of the quest doesn't reflect on the main plot, your increasing influence and mastery of Shouting creates the illusion that you are still making progress towards your ultimate goal, if only by making small steps that make you more of an equal to the demigod dragon antagonist.
We don't get that in F4. The main plot is that you have a positive ID on the murdering psychopath who killed your spouse and kidnapped your baby. You have a very simple goal, with a very simple solution. You know (or think you know) the who, the how, and the when, you just don't know the where. Looking for answers at Diamond City? That works. Enlisting Nick's help? Absolutely works.
Helping the Minutemen? Stops working by the time it turns into you completely shelving your personal quest to help build up a bunch of bases across the map just to be a good samaritan. Hell, it stops working the moment that the Minutemen - the volunteer militia driven by the desire to protect the innocent and make the commonwealth a safe place again - doesn't ask its people to at least keep an ear to the ground and keep you apprised of any rumors reflecting your story. ...And then we get the player cosplaying as the Silver Shroud to deliver vigilante justice around Goodneighbor? ...I'm sorry, was that originally for a different game? Because pausing your search for your has absolutely terrible implications for the protagonist's sense of priorities.
While we can certainly put forward the argument that Skyrim works more out of dumb luck than deliberate design, the simple fact is still that verisimilitude demands that Fallout 4's main quest have a sense of urgency and focus that Skyrim's didn't. Adding to the frustration is that it's
so damn easy to fix. Railroad the player in the first act and cap that off with the player seeming failing. They catch up to Kellogg, they see something that gives them reason to believe that Shaun is dead. Boom. All of a sudden they're rudderless. Through the machinations of fate, everything has been taken from them. Home, world, family, future.
All of a sudden, open world works. The gameplay is not only justified from an in-character perspective now, but mucking about in the barren wasteland of the Commonwealth becomes a practical application of them coping with their despair. Do they rage against the lawless chaos of the world, believing that the only way to prevent more tragedies like that from occurring in the future is to impose order with an iron fist (Brotherhood of Steel)? Do they try to move forward and do what little they can to help others who are trying to cope with this desolate, unforgiving world (Minutemen)?
Do they - and this would require introducing a certain synth child earlier - try to cope by latching onto a synth as a surrogate child and therefore devote themselves to the Railroad? Do they outright deny the reality before them and latch onto the Institute as a way to turn back the clock, deluding themselves that the pristine environs means that nothing bad actually happened?
This practically writes itself! And all I have done to enable it was introduce an early story gut-punch that makes the player feel justified in setting the main quest aside.