The truth on the ground is that it's just not that black and white. Unless you're from a disfavoured country or pretty unlucky, being asked for a ticket out when flying into HK or Singapore or KL say would be rare. Not to say it never happens, so the OP should as I've suggested be prepared with a backup course of action. But you can't paint a true picture in such absolute terms.
As I understand it, the OP's problem is not the arrival process, but about being accepted for travel by check-in staff at the origin.
As I said, being turned away at he arrival airport's passport control because of no documented onward travel is unlikely, especially so if you fit the agents idea of a "clean" arrival. But sometimes agents get out of bed on the wrong side .....
HOWEVER it would be unwise and unreasonable to expect the check-in agent at the origin airport to make a judgement call on what might happen at the immigration desks of the arrival airport: he/she will have instructions to refuse a passenger with no documented onward travel, or call a supervisor to resolve things.
As long as countries have an onward travel clause in their requirements for entry, airlines will protect themselves from fines, and the costs of flying passengers back to the flight's origin, by checking details of onward travel. It's not a difficult concept to grasp.