Which almost never happens outside of the minds of those convincing themselves to become a dashcammer.
Well there were over 4.3 million claims to just top 19 car insurance companies in the year 2013->2014. And we all anecdotally know a friend or family member that has been involved in a collision and the stress and hassle that more often than not comes along with it, especially if the other party decides to dig their heels in. Many of us will know first hand the difficulties faced when claiming, often not from the insurance company, but from the other party.
And your baseless prejudices to having a dashcam are really rather clear when you write things like "convincing themselves to become a dashcammer" as if it's a way of life or you're converting to a new religion or something! I really don't know how you can be so vehemently against someone literally having a small camera in their car in case they have an accident and they can show an insurer or the police exactly what happened so that it ends up with a fair conclusion. A dashcam can also record conversations immediately after an accident, which are great because it often happens that people change their story after they've had a chance to think about it, but before they write out the section on the claim form that says "Tell us what happened".
If you have someone run into the back of you unless they've got some proof you reversed into them at traffic lights for no apparent reason it's going against them.
I've seen it several times just on this one forum over the years, including a few years back when this exact scenario happened at a roundabout and his insurance were ready to settle 50-50. He then had to fight tooth and nail to get his insurance company to challenge this and send them videos showing that his car, which was an automatic, couldn't have rolled back because it has auto hold and when you take your foot off the brake, it starts trying to go forward.
In the context of 4,300,000 car insurance claims a year, this is a very specific and rare occurrence so let's try to not focus on this one scenario as being the sole reason to have a dashcam. However, the underlying principles are exactly the same - people lie AND misremember details particularly when they have had the shock of an accident and/or are trying to cover their arse! I have no idea why someone would be against a small, cheap, effective device that helps create a fairer decision when it comes to liability, but then this same person doesn't blink at spending £1100 per year on a warranty product which they will likely not even make use of.
Some European countries ban the use of dashcams. Why do you think that might be?
Because some countries are extremely protective of privacy laws and whereas the realism is that dashcams aren't there to infringe on people's privacy (as they drive about the public roads?), they may still be taken that way. Maybe they don't have the problems with false personal injury claims or drivers being less than honest about the events of a collision. Maybe they do but they haven't got to a stage where they have forced a rewriting of old laws created decades before dashcams were even thought about.
Some insurers offer a 10-20% discount on policies if the driver has a dashcam. Why do you think that might be?