In the 90s, SQ was in its third decade of operation, and was trying to do what EK/QR/TK are doing now. Trying to expand and become a global airline. There were objections along the way, and not just from Canada.
This article describes well what happened.
This one shows that other (regional) countries were having similar objections with SQ back in the 90s.
EK has been facing similar issues from the US3 and Canada in their 3rd and now 4th decade of operations.
They fly ATH-EWR, and rumor has it they want to launch DXB-Prague-US service. The results are going to be the same. If the route is launched, DL/UA/AA will cry foul and hope the government does something about it.
Fifth freedom flights will always attract the attention of the home carrier on either end. It's the name of the game. SQ shouldn't take it personally, and I'm sure they dont. Much of the execs in charge at SQ now were only infants back in the 90s. That stuff is history. With ULR planes and non stops SIN-YVR/YYZ more than doable today, if SQ wanted to serve either city, they could launch it tomorrow.
In reality, even if the Canadian government hadn't revoked SQ rights to fly SIN-AMS/VIE-YYZ, chances are those routes would have been terminated long ago anyways.
SIN-ICN-YVR was canceled in 2009 because it wasn't meeting expectations (the fact that the economy was tanking didn't help). YYZ would have suffered a similar fate.