Of course come from other countries see the life better here.
china, philippines, indonesia, malaysia, etc. which country quality of life can compare to Singapore?
Why I mention the 20 years is because they compare their lives back at home country and Singapore, of course Singapore is better by miles and they vote for pap because its the old guards of pap that build this better country, but they didn't go through enough years to see the decline of pap, hence the opposition is needed to keep at their toes. They also don't go through enough years as a proper Singaporean to know the dark side of Singapore.
Like you mentioned Singaporeans go overseas to nice places for few days then come back and compare and be unhappy, same for new citizens, they come from shythole environment in their country to Singapore, paradise. Then vote pap because its the only thing they hear back in their country. LKY effect is still strong with other countries.