But the OP wants a program where miles don't expire. SQ's KF program would probably be the absolute worst choice you could make. It is one incredibly horrible program even if the miles didn't expire, but they do. Absolutely worthless for the OP to even sign up for it.

There are some programs where miles don't expire as long as you have activity. UA's program is the first that comes to mind. BD also, which has a good program, but being BMI was just taken over by LH, if the program is combined into M&M, then it's likely miles will expire unless you fly enough to keep Gold/Senator status.