It's well known how courts/DAs here in the states pressure people to accept lesser charges to avoid the expense of a trial. There's no guarantee an innocent party will win at trial, and that risk of losing coupled with the cost of fighting can and does lead innocent parties to accept a plea bargain. The courts are run like a business, in that efficiency is a concern. Pragmatism outweighs idealism. The downside of taking away qualified immunity for police is the same as the downside of bringing every case to trial. Put simply, it would result in a more costly process, impeding the work of doling out punishment.