I'm hearing what you are saying - but we're writing fiction. Anything can happen.
I'm contracted to sell books and write the best book my memory and imagination will allow. My readers will judge me.
The ending is difficult - but there is usually a few paths to take. If the author makes shit up that doesn't seem logical or reasonble, then they deserve to be outed as 'Deus Ex Machina'
JJ
When you write fiction, you write your world within the rules you create for it. Then you stick to those rules. Your readers accept them as fact just as much as they would accept gravity on earth and a blue sky. You treat the world you create as though it's not fiction - because to the characters you create, it's the only reality they know.
There can be last minute saves pulled off effectively, but it HAS to be done within the confines of the mythology you create, otherwise anyone invested in the story will make a sour face, call foul, and post nasty things on the Internet.
A good last minute save, IMO, was the end of the first Jurassic Park movie. The T-Rex came in and ate the raptors - I don't think too many people were expecting that, but it was acceptable within the world the movie created. We knew T-Rex was on the loose; we knew he was an alpha dog predator who was going around eating other dinos, and the whole point of that movie (sort of) was that eventually the dinosaurs would get around to doing what it was dinosaurs were supposed to do. It wasn't like they had established that T-Rex had never been created on the island or that he was malformed in some way so that he couldn't attack and survive.
Then there's DeM...
Imagine that for say 3 LONG novels you have a writer who establishes certain rules for her universe. Let's say these rules are:
1. My characters don't play well with others in large numbers unless they only eat animals as opposed to humans.
2. My characters are so unbeleivably gorgeous no one can miss them.
3. My characters cannot procreate sexually.
4. My characters have a pact with a secondary set of characters forbidding ONE action; going against this pact will result in the death of all of my characters.
5. My characters must answer to a higher social body that only leaves Europe in extreme cases where eradication is necessary for punishment. One member of this social body has the ability to see the entire life of anyone - beginning to end - simply by touching them; he has touched at least one of my characters.
6. One of my characters is a precognitive telepath who can see her family at all times -- especially her two favorites: her brother and new sister in law; the ONLY time she can't look in on them if she wants is when one is in close proximity to their mortal enemy and therefore in mortal danger.
7. No one knows my characters exist as anything other than human.
8. It is agonizing to become one of my characters' species so that it requires writhing in agony for three days as though one's whole body were on fire.
9. One a character transforms from human to my other species, they are uncontrollable, insatiable, blood crazed fiends for at least a year.
10. There's a nifty love triangle built based on the fact that the two males involved are natural enemies.
Now, imagine that in book (LONG book) 4, this same writer just disregards all of those rules and in doing so undercuts all of the dramatic tension she's built through all of those other novels. She poises a massive all in battle and....
1. Well, the nice one asks nicely so suddenly there are allies.
2. A few hundred of them show up in a town so small they go ga-ga over EVERY single new face... but I don't think anyone noticed. (and they all had "sparkling" personalities to boot...)
3. But those MC's were too darn pretty not to have a perfect little rock hard bundle of terror.
4. Well, I guess it's ok if the character's old friends go out and hunt around the little town while they're visiting; the secondary characters can turn a few blind eyes. They're only honor bound to protect the humans from brutal, painful death... it's for a good cause after all. (that kid really is cute.)
5. The hierarchry makes a mass exodus from Europe. The leaders; the guard. (yikes, even their wives) AND... AND... they shake hands, shrug and walk away.
6. She lost contact with both of them (because of the rock hard bundle of terror)... and didn't think it was worth mentioning.
7. Except that one maid from an obscure tribal region who just happens to be cleaning the love nest and knows instantly what the MC is and how to fix things when they get out of hand.
8. Except that my female MC doesnt want the others to feel bad... so she keeps quiet.
9. Now she wouldn't be special or ladylike if she went all blood happy; she can control herself - it doesn't even take much effort. Now we all marvel and her wonderfulness.
10. Nevermind - he wasn't in love with you; it was the rock hard bundle of terror (that's ten minutes old) he wants.
11. While we're at it, lets just give supergirl a nice shiny telepathic shield that she can magically extend around her family/friends at will to make sure the bad guys can't hurt them at all. And while we're going down this road the bundle of terror gets to be a superchild who's not only a genius of mythic proportions, but grows superfast so she can marry her true love by the time she's two.
THATs Deus ex Machina.