The ending may actually be more true to Austen's change in style and increasing cynicism as her works and life progressed.
S&S and P&P are the "classic" love stories, written much earlier than Lady Susan, for example, which is much darker and less forgiving. Even Persuasion and Mansfield Park have darker themes in them. I think Sanditon is most definitely unfinished by Davies and it will probably end up with the classic ending (I am expecting an Edward Ferrars style release from Sydney's engagement).
I actually think Davies has done a decent job with the material, although I feel like there was too much packed into the last episode. It will be interesting to see if the ton make it into the plot as that would be distinctly non Austen. I hope there will be a second series.