Are the domestic lounges busier than international? That's the only way I could remotely see this make sense.

There are many times I've flown something like YYT-YYZ-SFO, with a decent YYZ connection, along with someone flying YYT-YYZ-YVR with a shorter connection, so I've wanted to stay domestic until they board their flight.

You're still subject to the 3 hour or connection rule, so this isn't resulting in more lounge time. If anything, there's more time spent walking to domestic, going upstairs, waiting to check in, etc., that this should actually result in slightly lower dwell time.

Obviously as SE it wouldn't impact me, since I'd have an arriving BP in those cases. But it still makes no sense.