Let's say a 50-year old has paid off their 25 year mortgage and their £50,000 house (1980 price) is now worth £500,000 (2005 price)
Wouldn't it be better to sell your house and move into rented accomodation and then spend the money on buy to let properties for a further 25 years? (rental income to be paid out of wages which previously were going to the mortgage.)
If you figure the current average house price is £170,000. Then you factor in a resonable 20% deposit for each house. You can have 14 properties instantaneously with a total value of £2,380,000
Then once the loans are paid in 2030 your £2m portfolio could be worth £16m (if you accept that house prices double every seven years) which would be enough to retire on!
Obviously this length of time will be able to ride out any house price collapses. Any difficulties in paying your rent can be offset by selling a property. And these calculations do not take into account rental income received for 25 years. (although they do not include void periods and CGT either!)
Although a bit simplistic in explanation, the concept seems entirely feasible to me.