1) it doesn't matter now because property became a lot more expensive during the period when interest only mortgages were given.theyve gone but the inflated house prices remain- easy lending to lock more people into the debt trap- usury based economic system.
If this is a problem for you rent. however, for whatever reason, if you want to buy a house and you don't have the money up front you will have to borrow it. It is unlikely that somebody will lend you enough money to buy a house without charging you interest unless they are your parents.
2) If the profit for the bank was on the house itself, not the money lent, then it wouldn't matter whether someone paid it after five years or 25 years. In fact, you'd be happy if someone borrowed money off of you and paid it off quicker than they had said they could. You get the money back quicker and get to use it for something else. And they've saved you from your money losing value to inflation.
Unless the house lost value. Banks provide loans because people want to buy houses. They also invest in property, but they might not want to invest in the property you want to buy. Also, assuming that the house has risen in value, how are the bank going to realise this gain if the house belongs to you and you are living in it?
3) but why should there be any prohibitiveness at all in making overpayments .... If the system of profit wasn't based on usury that is.
When you take out any fixed rate agreement you do so to avoid the risk. In return it is normal for the person bearing the risk to commit you to stick with your agreement for a set amount of time. If you could just opt out they would't offer you the deal.
if there wasn't this sense of fatalism that debt is inevitable and also a value system based on not having worth unless you have material possessions. (That we start kids on with 'Santa only brings presents for good children dear!') then more people would reflect on if there is another way for the system to be.
Or they could just read the small print before entering into an agreement.