Okay, folks. Hear me out.
Here’s a thread of why I think that Sam and Dean didn’t live a life of free will after they defeated Chuck in 15x19:

You remember 15x09 The Trap, when Chuck did show Sam their future? Every time they traveled further, they stopped on a date where vampires appear.
On 04/17/2020, Jody called the boys and Eileen, saying that Claire was k:lled by a nest of vampires.

On 11/03/2021, Sam and Dean talked about a case – talking about vampires.
And on 12/09/2022, both Sam and Dean were vamps.
Ultimately, Dean was murdered by those monsters.
Also in 15x09, right before Sam traveled to the last possible time, he had a short conversation with Chuck.

Sam: “No, the Dean I know… the Dean who raised me- he’d never give up, no matter how bad things got.”
Chuck: “Well, he does. He will. This is the truth, Sam”
Ultimately, Dean gave up.
Now let us take a look at 15x04 Atomic Monsters, the scenes with Becky and Chuck.

(Also, Sam and Dean worked a vampire case in the same episode.)
After Chuck wrote his first draft, Becky described it the same way as 15x19 aired.
After him writing the second draft, she’s terrified, horrified.

Saying the draft is “so…dark” and “What you did to Dean? What you did to Sam?” and “It’s awful! Horrible. It’s hopeless.”
And Chuck saying: “I can see it… ‘Supernatural: The End’. And the cover is just a gravestone that says ‘Winchester’.”

Of course, you can say anything could’ve happened in that draft. But at the end of that episode, in the very last seconds, we see Chuck continue writing and we see a zoom-in on the Funko Pops of Sam and Dean, underneath them a notebook (the journal) and right next to them a tree.

Clearly a foreshadowing to what happened in 15x20.
But why a hint to the 15x20 “free will” episode in a 15x04 episode where Chuck is still under control?
Because Chuck is still under control. He knew what happened in 15x19 because he wrote it in 15x04.

Before Jack absorbed God’s power, God told his vessel (the real Chuck, who’s still alive after all those years), that if he doesn’t play along his rules and makes the boys think it’s just God without his power, he’d kill him.

After all, there’s no archangel left to protect him anymore.
Jack's gestures show us that it's actually not Jack at all. His smile is different. His arm movements aren’t the same either.The left pic is a picture of Jack. The two pictures on the right are Jack and Chuck as God.

Also, why would Jack not rescue Cas from The Empty right after he brought back everyone else? He knows how to do it because Lucifer did it back in s14. Plus, he is his dad!?

ALSO, he was overwhelmed with ‘just’ being a Nephilim, with being ‘the spawn of Lucifer’. I’m pretty sure being God as a three-year-old kid is just as complicated. Why would he leave Sam and Dean?

And… what is it about heaven? Jack rebuilding it, Cas helping him?
Well, you can interpret a lot here, but what I’m writing right now is also just my theory: It was also just some scripted bs of Chuck.

Remember the filter they used for the Heaven scene? That’s the same filter for when Zachariah manipulated Dean in 04x17.
Secondly, the El Sol beer. It always appears in episodes when something happens that is simply not right. A hint that Chuck made Dean something up.

I don’t know…
…what Chuck did to Sam after Dean d-worded.
…if Sam actually lived a life or d-worded on a similar hunt, alone.
…if Dean was actually in Heaven.

It’s possible that (off-screen for us) Chuck (as Jack) appeared after their reunion, laughed at them, snapped his fingers and in the end, they are in Hell. I mean… It’s Chuck. Anything could happen.

Plus, a lot of interpretation, in the script we see many omitted scenes. What if those omitted scenes ended up in ‘Jack’ sitting at a computer, writing? Or him watching the boys do their stuff while he is invisible to them, watching his favorite show as it slowly comes to an end?

That's the end of my analysis. I hope I didn't break your heads as much as I did break mine.