This is Q we're talking about. He's not the kind of character who would sit down with Picard over a cup of tea and talk him through why changing his past would

This is Q we're talking about. He's not the kind of character who would sit down with Picard over a cup of tea and talk him through why changing his past would change who he is. Humbling Picard by showing him the folly of his wish to change the past, while he lay on his deathbed, by making him choose to get stabbed through the heart is definitely more his style. Also, I don't believe they portrayed Picard's "unremarkable" career negatively, except in Picard's own view (and Q's I guess).