Single pilot IFR operation is perfectly safe. But, as I've said before, it requires well equipped aircraft, pilots who are experienced and kept well trained and proficient, and infrastructure. A few places are serious about operating IFR, while too many others pretend to be IFR, but won't make the investment in those items to do it right.
Now add a second pilot. That will require a larger aircraft in order to operate IFR. It will also double a company's training requirement and budget. Consider that some places, right now, don't invest enough in keeping their pilots trained, and the practicality of 2 pilot IFR falls apart.
Will companies hire experienced pilots and essentially fly dual captain IFR? They would likely have to in order to reap real benefits. Or, would companies try to save money by hiring low time copilots? HAA doesn't fly enough for those copilots to get the experience they need to be good captains. Low time copilots will stay copilots for a loooong time.
There are places that do fly IFR in weather lower than just "not quite VFR", and do it safely on a regular basis. Those places also take IFR seriously.
Will having two pilots reduce mistakes and mishaps? Maybe, maybe not. Poor headwork and poor CRM are dependent on more than the number of pilots involved.