Sorry, your complaint is about them having no seats to sell, and you not wanting to pay for MLL access?

I don't think AA or DL J would get you into a lounge on transborder itinerary (if there's even a partner lounge in YUL TB).

So I'm not sure how AC is worse than the others.

So sure, if you want to connect on UA, and they have seats left, then yeah, you'd have MLL access.

But UA can just as easily be full in J as AC.