I suspect a lot of them are melted down for scrap. I've just been offered a £140 for an X-reg MOT failure Nissan Micra. A single bike won't be worth much but a few dozen will.
Is the water supply really so bad down your way ?!!There's a charity is there not that sends old refurbed bikes to Africa, for the folk out there it get to school and carry water and what not. The postman told me last week that Royal Mail are phasing out the RM bike fleet and sending all the old bikes to the charity that sends them to Africa.
I picked up one old bike out of a blokes van last week which was headed for the tip. I'm going to tart it up and use it.
I genuinely don't understand why you are having a pop at the OP?
I tend to keep bikes for a long time, too. I look after the maintenance, and whenever a part needs replacing then it gets replaced, normally by something better than standard spec. The only original bits on my nearly 4 year old Tricross are the frame, bars, forks, brakes, and seatpost! I don't see the point in "upgrading" by buying a completely new bike, when the manufacturers keep "downgrading" the spec of their bikes while increasing the price year on year.
As to where do all the old bikes go? I have heard a lot of them hang about the pubs in Airdrie .