What some people are hankering for is called "Nostelgia", they know that they cannot get it back & cannot reconcile to the modernity of Singapura of today. Added to this, SG does not IMO do enough to record, treasure & preserve it's past, i think LKY alluded to this about 10yrs ago in a ST interview, he said Singaporeans will not have a coherent identity for the next few generation or something to that affect. I guess he was anticipating letting in more immigrants/new blood. So i guess some Singaporeans will feel "out of place" in their own country & the need to migrate out.
Having spoken to lots of long term Singaporeans living abroad, they all have a similar opinion, the Singapore of their past where they grew up does not exists anymore, whenever they balek kampung to SG, they do not recognise it & do not enjoy their stay. Family & friends will die & even tastes of their favourite SG food have changed over the years, their connection to SG is severed even more. Perhaps at one time some might consider returning to SG in their old age, now they prefer to die & be buried in the countries they have moved to.
Singapore will not be in a situation where it has no more people, only monkeys.. becos it will always open it's doors to new migrants like it has over the last 2 centuries. Unless you consider new migrants into SG as monyets
All i can say is, regardless of which party politics you follow, governing Singapore as a island nation is a tough balancing act, it is unlike any other country with it's mix of people, social history & industry that i have come across. When i meet people on my travels, they are always curious about Singapore becos they see it as a vibrant city that recreates itself, a bit like the future cityscape from the film Bladerunner.