Oh man, I have *feelings* about endings. At least for me a bad ending can ruin a story telling experience that I had previously been enjoying (that may be extreme on my part but I will die on this hill). Buckle in for a rant!!
I think unsatisfying endings come down to a couple of things:
- not fulfilling the promises made either explicitly or implicitly at the beginning of the story (I'm stealing terms from Brandon Sanderson's lectures, but I think this sums it up the best).
- ending didn't tie back to the MC's growth or story's themes. I think these are the ones that often feel "unearned"
- ending was "too easy". It sounds like this is what happened to your thriller, OP. Unless the hero was supposed to learn they can't do it all alone, a "saved by the cavalry" ending often feels arbitrary. Lack of set up for the solution
- not enough denouement. I love the calm after the storm beat and *so many* stories skip it! Even a lot of beat sheets and craft books leave it off. It's probably the least egregious sin, and it's unlikely to "ruin" and ending, but I hate feeling like I missed something at the end. I want to see the "new normal" after the dust settles.
To be fair, endings really are difficult. They rely so heavily on everything that came before. You have to weave all the threads together, come up with a clever solution to the problem (after all if the problem were easy to resolve, there'd be no story), and oh, make it flashy too! If anything was lacking in the first 75% of the story, it's going to come back to bite you in the end. The writer also has to draw conclusions about whatever themes or character journeys were set up before. That can be tricky and occasionally fraught depending on the subject matter.
I wish I could remember who made this observation originally, but it does bear repeating that endings are often the part of a story writers get the least practice with. Beginnings? Tons of how-to's, advice, and they are easy to crit without reading a whole book. Endings not so much. A lot of projects never even get an ending if the writer trunks them! I also wonder if endings get the short end of the stick in editing because everyone is tired or time-crunched or some combination by the time they get there. Writers and editors are only human, after all. Idk, that's more speculation on my part.
Okay, rant over. Endings are tricky and arguably the most important part of a story and also the hardest to practice. Lucky us
(though this does make me appreciate good endings all the more!)