I am coming in a bit late into this discussion. I think the whole thing has been discussed to death, but I would like to point out three things
1. Some people will never buy new cars - they will always buy second hand cards. A 3 year old second hand car with low mileage will always be attractive for this group. PCP allows a steady stream of these cars to the market and should keep the prices 'reasonable'. I don't believe the second hand car market will tank, although there may be some value in certain make/models. I can imagine if VW had another scandal in 2 years time, the value of a second hand VW might drop as an example. I don't see there to be 'bargain' 2nd hand Audi's or BMW's out there, as these will always have a 'floor' price.
2. Some people will always buy a new car where they can. They are willing to take the hit financially to have the new car, whether its PCP or not. For these people, PCP is just another form of lending, or in the case of 0% finance, a means of managing cash flow. This group will be buying the car - PCP available or not.
3. The 3rd group are the ones who will fall for the PCP deals, and potentially get caught by then. They have 10-15k to spend on a car, so instead of buying a second hand one, they are attracted by the PCP offer and buy a new one. In 3 years time, things may not work out the way they expected, and end up without a car or struggling to work out how to keep the car they have. They don't realise that the new 40k car has to be paid for some way, and the 15k deposit and 10k repayments still leaves a balance to be paid. These are the ones who have changed their behaviour because of the PCP deal.
Most people will fall into group 1 & 2 - but a few will fall into group 3. This is the group being referred to in the above posts where people are nervous about the scheme, not the likes of Blackrock1 who will buy the new car no matter what.