If we use our CPF OA to pay for our flat, and at age 65/70, we are still able to meet BRS/FRS, does NOT refunding the amount used and accrued interest affect our lifetime payout in any way?

If you sell your property before age 55, you have to 'return' your CPF account the principal used as well as the accured interest.

If you sell your property after age 55, you only need to have full retirement sum in your SA n you can withdraw all balance in cash.

If you didn't sell your property, you only need to have BRS n you are free to withdraw all cpf money. (MA can't lah)

Not doing a housing refund only affects you when you sell your house. When you sell your house, you will need to return the minimum of (the amount used + accrued interest) versus ("profits" from selling your house) back to your CPF OA.

The longer you take to sell your house, the less likely you will get any physical cash from selling the house because most if not all of it goes back to your CPF OA.

If you are not selling your house, then the accrued interest just increases... that's all.