I'll start by saying I'm not a homeowner.
Surely something is only worth what someone is willing to and is able to pay. So if people are only able to pay for £140k of the £260k asking price surely the property is only worth £140k? But no! There is money to be scammed here...so let's make people pay rent on top of their mortgage even though they will never own that % of the property!
It smacks as one big con to me.