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.

I see, interesting! Agreed, on such a system it becomes more of a gamble to pick what might be best long term.

In the USA there is no penalty to pay off a loan and refinance (there might be exceptions but never seen one) so you can do it at any time as frequently as you like and keep ratcheting the rates down.

There's also not such a huge difference. Looking at zillow today, 30-year fixed shows 4.95% and a 7-year adjustable at 4.81%