Indeed. I was able to switch to every CDG-SIN flight (both midday AF182 and evening AF256) over the following four days or so. I was travelling on one of the days where the evening flight was the only one operating, meaning leaving at midday wasn't an option, meaning all the flights offered would've resulted in me arriving many, many hours after my connecting flight had left. I'd like to think a call to the PSL would've eventually fixed this had I accepted the offer, but for 150€ I had no interest in taking that risk
That's incredibly worrying.
They should not be offering this "option" to those with onward connections because - as you say - they clearly aren't even considering the disruption that this would cause if such a customer unwittingly took up this offer and ended up losing their SQ flights...
It seems a very basic flaw in their coding that they don't limit these offers only to O&D passengers and - if offered to connection passengers at all - only to those on the final segment of an inbound/outbound part of a connecting itinerary such that selecting any of the choices offered doesn't "break" the logic of the ticket.