The setting was serious from the very beginning. the seriousness was subtle, but it had always been serious behind the comedy and ecchi.
Yeah, like the very 1st chapter / couple of pages shoulda set the tone: guy trains for # years to be a hunter, & becomes the Ace in less than 2 years. As others retire, he ends up looking back & basically wants to retire / take a break, but since there's basically no one to take his place, he has to train someone himself.... Problem here is that they are having him train "misfits" (for lack of better words), that if anyone else tries to train, would prob get them all killed.
To add insult to injury, his achievements aren't recognized, because it's not news worthy, "behind the scenes", or attributed to others.