I have had the same doubts about the essential idea of the story--that the Loisels almost immediately decide to replace the lost necklace rather than having Mathilde go to Mme. Forestier and confess frankly that she lost it. This seems ""heroic" but also stupid. It is largely the husband's idea to try to replace the necklace regardless of the terrible cost. When all his efforts to find the lost necklace fail,
"You must write to your friend," said he, "that you have broken the clasp of her necklace and that you are having it mended. That will give us time to turn round."
She wrote at his dictation.
At the end of a week they had lost all hope. Loisel, who had aged five years, declared:
"We must consider how to replace that ornament."
Not only is the cost of a diamond necklace far beyond their means, but they can't even be sure they can procure a necklace that will look like the original. Expensive necklaces must usually be of unique designs. Maupassant has Mathilde go to the jeweler whose name is on the case in which the original necklace was contained. If this jeweler had been the one who sold Mme. Forestier the necklace, he would, of course, have informed him that it was only made of imitation diamonds. But the jeweler tells her:
"It was not I, madame, who sold that necklace; I must simply have furnished the case."
So they truly have an heroic task. They must duplicate the lost necklace without having the original for comparison, and they must pay for the replacement with money they haven't got. In the meantime they must try to stall Mme. Forestier, who may be wondering why Mathilde hasn't returned her necklace. Any rational reader would ask: "Why don't they just tell the truth and offer to pay Mme. Forestier in installments?"
The only possible explanation is that M. Loisel is concerned about his job and his future career. He may be afraid of being accused of stealing the presumably valuable necklace by pretending that his wife lost it. They are both afraid that in a worst-case scenario Mme. Forestier might report them to the police. They probably couldn't be arrested, but there could be a scandal. It might even get into the newspapers. The police might make inquiries about Loisel's character at the Ministry of Public Instruction. The scandal could reflect on the Ministry because the Loisels were guests at the palace of the Ministry the night the necklace was lost. The necklace was a cheap imitation, but the Loisels do not know that. The replacement cost them thirty-six thousand francs.
There are weaknesses in Maupassant's story. There are almost always some weaknesses in even the best short stories. But a gifted writer like Maupassant can usually manage to cover them up. We go along with the Loisels in our imaginations even though we can't help thinking that we would have gone directly to Mme. Forestier and told her the blunt truth. The Loisels are heroic, but being heroic does not necessarily mean being intelligent.