I didn't correctly read your comment sorry. I don't know about the US system, but here the fixes normally do have a lock-in typically on a ratchet system where they are expensive to exit in the early part of the loan and cheaper later. For example one I just checked charges you 5% of the loan to exit any time in the first 5 years and then 3% to exit any time in the next 5 years. There is also a significant discrepancy in the interest rates charged for different term lengths e.g. the offer presented may be 3.7% for a lifetime fix or 2.4% for a 5 year. So you're paying 1.3% extra for the lifetime fix right out of the gate. So the gamble is will 5 year fixes cost more than 3.7% in 5 years time? If not, then you won on the 5 year fix.