Many old sites will stop working (my first site was done in flash) as well as many games that are still heavily played today by millions of people. Also flash IDE provides a good introduction to programming for self-taught kids these days: many of them still do their first code in flash after clicking on "that strange icon next to photoshop".
Overall this is a good example of prolonged trusting a binary blob. IMO we will always tend to do what is more comfortable and we should strive for openness and transparency in the tools that most people rely for everyday.
The problem persists as long as there are people installing the plugin or "enabling" it.
We need a real open-source alternative to flash player.