When I first read Jane Eyre as a teenager, I was simply happy that a Gothic novel had a "happy" ending. However, now as a more mature reader (I hope!), it is easy to see how the novel's conclusion is problematic for many readers and critics.
First, some critics argue that even as Jane Eyre provides a realistic look into some of the social issues from the Victorian era, it relies more on improbability. George Henry Lewes, a well-known critic from the time period, argues that the novel is too melodramatic, and no one could be expected to believe that a man could "hide" an insane wife in his home for years. Many of the unrealistic elements of the novel do occur at the end. For example, Rochester just happens to get his sight back in one eye after he and Jane are married for two years, and St. John symbolically becomes a literal saint by dying in the "service of his Master and Lord." Thus, if one is expecting a cynical reality at the novel's end, it is undeniably disappointing to the reader. Similarly, if one reads Jane Eyre as a feminist work, the ending is troublesome.
That being said, I still read Jane Eyre as a Gothic novel, and from that literary perspective, the melodrama of Rochester's blindness and the destruction of Thornfield work well.
While the ending of Jane Eyre is a little too contrived, is this contrivance not typical of Victorian novels? Certainly, Dickens's use of dopplegangers and coincidences as well as his excessive repetitions and stylistic decoration were excused by his avid readers. Readers must be careful to not judge works solely by contemporary standards, for styles change. After all, authors do try to sell their novels, and what is the trend in literature must be taken into consideration.
One consideration of Jane Eyre is perhaps, the symbolism of eyesight that the reader must consider rather than its credibility. For, Jane's visual imagination is what attracts Mr. Rochester and reveals her singular intelligence to him. Also symbolic is Bertha's blinded eyes and Rochester's temporary blinding that give great significance to the role of sight in Bronte's novel.
I think the ending of Jane Eyre is just a little too neat and contrived, of course. I do like the picture we get of Rochester doing the right thing by trying to save his wife, despite the fact that he would be better served if she had died. And, to be perfectly honest, I'm thankful Jane finally gets away from the depressing sidetrack she was on with St. John. The fact that Rochester regains his sight is the cherry on the top--the romantics love it, for the rest of us it's just too much.