The harm is that people may start booking economy tickets, knowing they can consistently upgrade to business for a fraction of the retail price at the airport. And KLM was often selling the day of departure upgrades first come first served *before* processing waitlisted GUC instruments, thereby skipping over any sort of upgrade priority list. Delta doesn’t offer a departure day buy up option at any other airport for their own flights - it was only a KLM program at AMS.

I know that I had previously changed my booking patterns to account for the easy availability of day of departure upgrades at AMS, and I suspect I wasn’t the only one.