I'm glad you've had good experiences, antirealist, but I have definitely witnessed people of various citizenships (including non-Canadians) suffer terrible consequences after having failed to return an I-94. I'd take care of the problem now, before the next trip to the US, rather than risk an adverse event upon next attempted re-entry.

Incidentally, I often have a multi-entry I-94 in my passport, and I face the opposite problem: over-eager gate-agents who remove the I-94 from my passport even though it needs to stay there. On one occasion, a gate agent removed my husband's I-94, and my husband threatened that he refused to board the plane unless the gate agent returned his I-94. I could only chuckle! I mean, why would the gate agent care whether or not my husband boarded the plane?! The crew would gladly have left without him onboard! After the intensity of the discussion was brought down a notch, the I-94 was returned, and everyone continued along their merry ways.