Hard disagree to all flights operating from EWR. Without starting yet another EWR vs JFK thread (JFK has better transit, yada yada yada), anyone east of Manhattan isn't going to want to go to EWR, so SQ is better off splitting service. Indeed, SQ operates two flights out of JFK (including SQ25 via FRA) vs just 1 out of EWR, so clearly they see value in at least splitting service, if not favoring JFK over EWR. (The lack of UA transfers at JFK doesn't seem to bother other *A carriers, including AI, AV, CA, LH, LO, LX, MS, NH, OS, and SN.)

While lounges clearly aren't the deciding factor for the majority of passengers, I do agree the JFK T4 situation isn't ideal. The best option would be for SQ to pay Delta and send SQ23 passengers to the D1 lounge or one of the SkyClubs, but that seems unlikely. (SQ25 is probably too early and would overlap with the tail end of the TATL peak.)

Alternately, if the timings of SQ21 and SQ23 were swapped, a 10:30 JFK departure could use the VS lounge for all passengers. (VS26 leaves around 8am, then the lounge switches to Priority Pass until the evening TATLs.) Though this might just cause the same problem at EWR with the BA lounge needed for BA passengers in the evening.

The best option might be to move all service to JFK and then use 3x daily as an excuse to build an SQ lounge somewhere, but I'm not sure where the space for that would come from. Or as long as we're dreaming, maybe do some *A terminal consolidation and get a joint lounge out of it.