I get all of this, but my point is that it's all a bit silly. Hopefully by June they will have removed ALL of this stuff, and even if someone was denied boarding, it would just be more revenue for Singapore for an unplanned stopover, really. Almost everyone in the world has the ability to enter Singapore without a visa in the pre-Covid setup and I'm saying I hope they go back to this sooner rather than later, given the dubious public health benefit of many of their current rules.
It's really not true that almost everyone could enter Singapore without a visa pre-COVID. Citizens of lots of countries require visas, including people from India and China. Obviously those two countries on their own account for something like 35% of the global population, and they also account for a lot of travel to/from Singapore (in normal times).
FWIW, even pre COVID, if you showed up at the airport with a one way booking to SIN only, I suspect you would only be allowed to board if the airline was satisfied that you had the right to enter Singapore. I'm not sure if they would ever have accepted "I have a separate onward flight booked with another airline, and I'm going to visit the transfer desk on arrival". I certainly wouldn't want to rely on that going smoothly anyway.