I think part of the problem is that although, yes, it is a marketing gimmick, it also responds to a very real need to allow extra space to either move baby seats in and out of cars, or to safely fasten children (often up to the age of 12 if they are small) into car seats.
During these halcyon days of yore when we all managed without them, there was also no law stating that we had to safely restrain our children in what are often very large bulky car seats, many of which are fiddly to fasten without being able to fully open the door and lean in sideways.
Of course we all manage without them, but I have to admit to there being several occasions when i've been parked in a normal size parking space and had, by necessity, my car door pushed right up against the side of the 4x4 parked next to me, just to allow enough room to get my DD or DS safely strapped in. I wasn't happy about it, and I doubt the other car owner would have if they'd seen it, but I had no other choice but to turf my unpredictable autistic toddler DD and baby DS out of the car while I reversed out into the middle of the car park to create more space!
That's not down to me being difficult, but purely to the fact that many car parks use old outdated dimensions to determine their spaces, that don't tally with the increasingly large sizes of the vehicles we drive.
I wouldn't sign this petition as I do think it's entitled and unrealistic, but I would happily sign a petition to rethink and revise the standard width of car park spaces so that P&C spaces are no longer needed.