No, I don't think your expectations were too high. A major change such as the lead in an immensely popular series is bound to have some teething problems in trying to re-boot itself. When John Nettles left the series they had a huge hole to fill, not just in the lead character, but the familial cast of Cully and Joyce as well. Over the years the stories involved fewer and fewer cast members until Tom, his DS, and Tom's family became the prominent players. The initial episodes (series 5 in N. America) were crafted by Tony Horowitz from the original Graham novels and owed much to the extensive cast of village "quirks" which were wonderfully chosen and directed. As the years rolled on certain economies had to be recognized and the writers very ably carried on with fewer characters and focused much more on Barnaby, his family, and his Sergeant. Trying to re-create that familiarity and ambiance might be a "bridge too far", particularly for hard core fans that just aren't going to accept the reality that John Nettles has retired.
Regarding the pricing issue; I note that "Lewis Series 5" is now selling for less than $19 on amazon when just a few months ago it was priced at over $30. Realistic reviews such as this one will only speed that process up for additional releases of the new Midsomer Murders.