How many aircrafts were going from Zurich to Montreal or from any other departure point within Europe at that time in their airspace? Furthermore, how many of them were climbing directly to FL430 in their airspace and then fly on a random route across the Atlantic to Montreal? Well I can assure you that we never saw anyone near us both laterally and vertically (certainly not above FL400) for the entire flight that would have overloaded their airspace.
This was possibly the best part of your slightly mental rant. It matters not a jot if there were 70 aircraft from Zurich to Montreal or if you were the only one. The reason for a slot change will be a regulation in at least one of the sectors you are due to fly through on your flight plan. It could be in the UK if you were going that way, it could have been in France or it could be that the first en-route sector out of Zurich. It doesn't matter if you wanted big numbers that no one else can reach, you've still got to get through congested and busy airspace to get there.
It means that everyone to any airport that has filed through that bit of airspace will get the delay - we're not just picking on you, although with your tin hat mentality you probably think we are. There's nothing to stop you refiling around the airspace with the regulation.