If you manage it in a single day, a.k.a.. no long rest in between, the scenario would play out as if you had gone there directly. After all BG3 unfortunately has no way of measuring time besides days.
Again... "too long" would be after a long rest. And it would start by either noticing the gnolls attacking the cave or by getting the journal entry about finding the survivors.
So ...
Basicaly unless your character would get litteraly to that place, then turn around and say "nah screw this, i will go explore some more and return later" ... and then they would Long Rest ...
What exactly would change? O_o
Bcs i cant help the feeling that the answer is: Nothing!
The situation with Arabella and Kagha makes for a bad comparison though, since "failure" there is determined by a random die roll rather than by the player's choice
True ...
But the point was difference between "failure" and "failure that makes me feel frustrated" ... i gues i should have also include example of "failure that still feels good" tho.
Okey, take different example ...
Grymforge.
I fighted Duergars on ship, ended in quite bad shape ... but arived there.
Duergars were ... well, not exactly friendly, but at least non-hostile.
So i arived to the digside, where they all send me and find out that Nere was caved in ... i also learned about some explosives ... so i went to search for them.
I decided to go searching to the left ... i met group of Duergars that were whiping Rothe, in order to clear another cavein ...
I tought cool ... maybe if i help them, they will help me save Nere ... well, they attacked me ... i was in not-so-perfect shape allready ... so i Long Rest.
Since i cleared the way, i searched it ... bad luck, triggered some traps, fighted some merregon and few hell pigs ... another Long Rest.
Finaly i got in right direction, i have found little Gnome, decided to push my luck ... failed my roll ... ended up totally screwed ... Long Rest.
On my way back i have found another room with explosives ... so i gather those and returned to completely empty hall, where i cleaned cave in just to find out lifeless Nere body.
This isnt matter of any RNG ... it was my Long Rests, someone in Larian decided that limit it to 3 is enough ... it was not for me.
And still, it sucks.

You can't please everyone. Someone is always going to be disappointed by whichever method they use.
Exactly ...
So they can aswell keep it as it is. xD
As for people who'll try to break the system on purpose, let them. They paid the same money I did for the game, if their idea of fun is to see whether they can break the system more power to them.
Wich perfectly aply to both my original point, and what i said just one quote abowe ...
Personaly i see no reason to return to Waukeen's Rest, just to withness that "its still burning" ... to me it is breaking the system. (Figuratively speaking.)
So i say ...
As for people who'll try to break the system on purpose, let them.

I was 2 map squares away from the Vault. Was it frustrating at that moment? Sure.
Then you should understand.

//Edit:
And I agree that there's no reason to show the building burning from the telescope.
Personaly i have no problem with seeing Waukeen's Rest allready on fire ...
I dont remember it completely, but i have the feeling that it wasnt exactly clear to say *what* is burning there.
What triggers me much more about the telescope is that it have different zoom for every scene where we can see small dragon that have barely size of our palm ... then we can see whole nautiloid tentacle ... and then we can see roof of Waukeen's Rest so close so we can count Roof Coverings (and that one is really something ... that is like 1200x magnified, isnt it?).
And much worse for me is the fact that to see what we DO see in that telescope, unless i remember it incorectly, we would have to be floating somewhere above that bridge Kithrak destroy little later.
Since we are watching from South +/- ... but the image is taken from West, or maybe even North/West.