The issue with that is that none of the other movies or specials or serials were ever building upto to it. This messaged was shoehorned into the third movie at the last second because Dean wanted the same ending at the books.
There was no buildup to this, no forshadowing.
It's with unity that the first villain was defeated, it's with unity that toothless broke the mind control, it's with unity that berk started growing as much as it did. "Seperating from the dragons" was not the only solution. They could have moved to a bigger island, they could have hid, they could have found a more defensible position.
Your post here explains the movie, but it doesen't explain why the movie doesen't contradict the other ones. It's obviously clear that this came out of the blue and was never planned for. Again, I direct you to the very final speech of the second movie:
"Those who attacked us are relentless and crazy. But those who stopped them? Oh, even more so! We may be small in numbers, but we stand for something bigger than anything the world can pit against us. We are the voice of peace. And bit by bit, we will change this world. You see, we have something they don't. Oh sure, they have armies and they have armadas... But we... we have... OUR DRAGONS!"
Does this really feel like it's leading up to a seperation? Does this seem like forshadowing for "Yeah we give up."?
It doesen't, because none of this was forshadowed. The movies have always been about unity giving strength. The heroes have defeated far stronger villains before, this guy didn't merit seperation.
You keep saying "The message is about letting go", but the thing is, that messaged was executed really, really badly, because it fits with nothing we've seen so far.
In order for a need to "let go" to exist in the first place, there needs to be an actual reason for it to happen, when there wasn't one.
The last of his kind but with a heart of gold.
Speaking of the "last of his kind" there's another massive issue with that.
This personally bothers me the most, why was the biggest hook of the series ditched? All movies, the specials and all serials teased us with finding out about the night furies.
Even the second movie ended with that hook when hiccup told toothless they might find more of them, and it turns out they're all dead and toothless is the last one? What? (Dean confirmed toothless is litterally the last night fury ever)
Why was the whole "night furies are completely extinct, light furies are a different subspecies" thing not given more weight and just relegated to a few lines on the side? Night furies were the biggest mystery of the entire franchise and the anwser to that mystery is "theyre all dead lol"? Why was that even nessesary in the first place?
They were teased in the first movie, in the second movie, in all the specials, even in the series multiple times, they practically the overarching hook of the entire franchise.
The concusion of their mystery being "they all died offscreen while you weren't looking" is so weird, it litterally feels like a last second change, not even a change, just a completely handwave of "forget about it". Why have all this build up over so many years only to never use it on anything?