While it's true that SQ miles expire and SQ is very stingy, there are a few more considerations at play here. First, there is the issue of UA's financial health. UA is in the shakiest health of the major US carriers at this point. If they go under, your miles go under with you (not trying to fearmonger, but this should be a legitimate concern). As an option, you could credit to a different *A carrier.

You say you primarily fly SQ. Well, if you fly SQ, and credit UA, you cannot use your miles for upgrades on SQ. You will not earn an elite tier bonus flying on SQ (meaning if you have 2P [Premier] status on UA, you only earn the 25% mileage bonus on UA and US flights). So if you fly 60,000 BIS miles on SQ, UA would give you 60,000 miles, SQ would give you 75,000 miles. Couple that with SQ's 15% discount for booking awards online, and you should earn rewards quicker with SQ. And generally, award availability is much better for Krisflyer members than other *A members.