Ah. You're talking about a mortgage that covers the entire amortization period. In this situation, I can understand why they'd have to increase the payments if the interest rates change.
However, I've never met anybody who has agreed to a 30-year term. That is extremely risky, because you're locked out of re-negotiating for better terms down the line.
Ah! I see the issue, then.
In the States, thirty-year fixed-rate mortgages are probably still more common than not. But almost all mortgages allow for pre-payment (in fact, I think they are required by law to allow that) so if you can lay your hands on a quarter million in cash, you can just pay it off here-and-now.
Which makes it relatively easy to refinance; you just borrow the quarter million in cash on whatever the new mortgage lenders terms are and pay off the old one.
Similarly, thirty-year ARMs are fairly popular -- they will typically come with a lifetime interest cap, so you are guaranteed that the interest will never rise above (some obscenely large nominal sum).