The CBI series has never been about a thrilling plot per se. It is always a focus on the investigation and the ways in which Sethurama Iyer solve the crime along with team work of Chacko, Vikram and the latter coming in comedic avatars. Thrilling Mammotty films more plot driven are Utharam, The Truth and Ee Thanutha Velupankalathu.
Unfortunately such a film (CBI 5) is less likely to appeal to the younger generation who are more for dark themed movies with gore and plot thrills. A film like Kurup will be more appealing, and Kurup was not at all liked by anyone in my circle who complained the investigation part was poorly handled (Their Comparison was to CBI Series).
This disconnect between generations is showing in the contrasting ways in which these sort of films are being received and the differing reviews even on this forum.