SQ miles expire in chunks based on quarters, I think, and it happens regardless of activity over periods of time, but if you do use them, it is a good program if you fly on their airline. My brother uses them and simply uses the soon to expire miles on upgrades! He said having UA miles but flying on SQ did not really help him in this area and he could not earn some of the bonuses until he decided to join SQ and earn and burn there. While they are partners, it appears that having miles ON them is far more effective. Just fly a lot and you should be fine. If you cannot, then you can sometimes request a one time extension for miles to not expire, and if you wish, you could always set up an award ticket for your mom or something to fly on them or one of their partners. Last I checked, DELTA was still a partner. It is, I think, one of the last places where DL and UA are still partly connected, as they were oh so long ago.

I do not know what to say about leaving AA. I like their miles in my account because they are really still the easiest to redeem and they are the only airline I know of who still lets you hold an award ticket for weeks on end without purchase. It is because of this that if you did find someone who ever wanted to trade awards, AA ones seem to be very valuable and you would have the upper hand in such a trade! (this is between friends or family or co workers, etc. No bartering of money!)

I also like that if you wish to fly say, to Aruba direct from BOS, you can use AA miles to get that ticket as close as 2 months out lots of times!

I would hold on to them, but maybe burn up a few or add a few more to get the account level to equal an award level of some flight you think you will take some time. That way, when the time comes, it's easy and no running around trying to collect miles or waste ones left over is ever going to happen to you.

good luck!
MM