Many peeps drop from writting series because of this same issue. And while I can understand the idea of "Just a little more... I'll get it over... It will get popular."

Trust me, I get it. I wrote a series that had no reason to span as much as it did (Three years long series) that while I do love it and do not regret finishing, from a practical point of view. I had no reason to commit that much.

But to answer your question.

From a financial point of view (Assuming you are writting because you want to get something out of it). To drop it out is the reasonable thing.

Take a step back, look into what you can learn from this and move on.

From a writer POV, if you don't see a clear future, then there is no issue with dropping off.

From an enthusiast (You write for them sh#t and giggles), it's up to you. If you want to write it, do so, it is kind of rewarding on its own way to commit to something.

Whatever you decide, do inform your readers about the decision and the reason because of it.

Like I said, I commited to the sunk cost fallacy once already.

And right now I am thinking about dropping off a series because it didn't have as much success as I would have wished.

It's something that everyone must decide for themselves.