A critical with the result of "pierces heart" would be applied as stabbing through the hand/paw of the dragon. Damage would be lessened, but still apply. In this way, the dragon still gains an advantage by not being instantly dead. Bleeding, penalty to activity for the wound, some stun for the wound.
OTOH, it is only a lack of imagination to see our fighter ducking under the paw and rushing in to deliver the killing blow.
Tomato, tomatoe. RM is not a HP system and the 10 second long melee rounds are provide the system the romance it needs to make the combat work.
OTOH, if you want REALISM, no human will ever battle a 5+ ton dragon and beat it in melee.
Problem with realism, for most it is not very heroic. So we aim for simulation instead.