So adding more responsibilities to police to now also get expert level training in determining causes of fires and finding clues in a fire incident. That's kind of how we got here. Where the police are a panacea making them less effective and more corrupt since they are an even bigger keystone.
Separating out duties to experts is more effective. Let the fire department investigate fires and then pass on the information for the police to secure the suspect/s and follow the justice system. Same with mental health emergency cases. More social workers and experts dealing with a variety of mental disorders will be better to work people in crisis since they are trained for that.