I think a lot of people on here don't understand the time value of money.
Whatmeworry - let me correct your figures, assuming 5% interest rate:
Assume you buy a house today for £100,000 at 100% moortgage of £100,000.
Assume joint salaries are £33,000 to cover it.
Assume inflation is c 5% and both house and salary rise in line with it
A repayment mortgage will want you to pay £100000/25 = £4000 pa, on top of interest, ie c 12% of your income now per annum when you are yong and poor.
A repayment mortgage will see you back pay £7k per year, inc interest, totalling £177k over 25 years.
In 25 years time, the house will be worth between £300,000 and £400,000
The Mortgage will still be £100,000
If you sell the house to downsize you pocket house sales value less £100,000 mortgage.
The mortgage will be zero and you pocket the full value.
If you pay it off in the last 10 years you need c 10% of your salary for only 10 years.
If you pay it off interest only you pay approx £5k a year for 15 years, then £12k for 10 years, totalling close to £200k.
Your Salary will be between £100,000 and £133,000 in 25 years time.
Assuming this is right, on repayment, you will start by paying 21% of your income, going down to 7% towards the end. On IO, you start off at 15% of your income, going down to 12% in the last 10 years. This might feel more smoothed out, but you will have a huge hike of your costs towards the end and you will spend more than £23k extra over the total life time of the mortgage.
This might be right for some people, but I don't think your figures prove the case for IO mortgages at all. You still pay a lot of interest in the early years and then have to pay off a huge chunk at once, which not everyone can plan for.