In certain respects, we can't really know with certainty how Mathilde Loisel's life would have continued had she not lost the necklace. These kinds of counterfactuals, after all, are speculative exercises. This is especially the case as we look further forward into time. We can certainly make reasoned assumptions on the short term, but if we were to extend this analysis along a longer time frame (for example, something like ten or twenty years into her future), the exercise begins to become a purely fictional one.
In any case, the key thing to consider with a question like this, is the degree to which this experience has disrupted and ruined Mathilde's life, throwing the Loisels into heavy debt and poverty. Certainly, had she not lost the necklace, this life-altering experience would not have unfolded (and, you can assume, she'd have continued to live much in the manner as she had before).
As "The Necklace" opens, Mathilde is defined in terms of her vanity and pretensions. She believes she is entitled towards a loftier lifestyle than she and her husband can actually support. It is for this reason she borrows the necklace to begin with: to create a false image of wealth. In this respect, even before losing the necklace and falling into real poverty, Mathilde Loisel is unhappy with her life, made miserable by her own pretensions and frustrations. As traumatic as her fall into poverty was, you should also recognize the role this experience played in breaking those illusions.
Thus, at least in the short term, we can imagine that, had she not lost the necklace, life for Mathilde would have continued much as it had before. She'd have continued in her comfortable lifestyle as a bureaucrat's wife, while remaining frustrated and miserable that she couldn't have occupied a loftier place in the social hierarchy. Moving beyond this, however, years and decades into the future, makes for more difficult speculation.