I think a lot of people on here don't understand the time value of money.
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.
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.
Your Salary will be between £100,000 and £133,000 in 25 years time.
If you pay it off in the last 10 years you need c 10% of your salary for only 10 years.
And that is assuming neither house prices nor salaries rise due to other causes.
IO is not irrational, even if you never plan to pay it back, as you can see.