Not a fan of this rule. A bit unethical, you are clearly preventing someone from having a job and earning money on the basis that you technically own them...
Not to mention that you are hurting a player development, and potential future earnings.
That's like working for McDonald for 2 years, and have some sort of clause that prevent you from going to school and earn a degree.
Entry level slides still pay out signing bonuses. CHL players also get paid on top of that. Additionally, the CHL teams paid to train and develop these players and give them the platform to get drafted. If you start allowing every 19 year old to jump to the AHL you basically kill the CHL and then the pipeline ends. It would be fine if teams were allowed one exception every so many years, but even then I'd only support it with some additional requirements like statistical anomalies and NHL compensation to the CHL team, and it counting towards the cap regardless of whether the player is in the minors or not.
Your argument is basically that NHL teams should own the players after they're drafted, which is pretty hypocritical to me. The rule literally exists to sustain the CHL, the top players every year are 19-20 year old players. You're also denying younger players the chance to play against a guy as dominant as Dumais to improve themselves as they become draft eligible. A 16-18 year old league would be vastly inferior for developing those players into NHLers. You're basically suggesting neutering the Canadian development system that has produced 80% of historical NHLers.
Plus Dumais didn't work on his defensive game and his numbers were largely dunking on non-playoff teams (Columbus themselves shared an infographic near the end of the season where he was something like 3.5 PPG against the worst team in the league and 0.9 against the best). Halifax's mid-season acquisitions were an injection of talent and likely elevates Dumais' numbers to the inflated number he got to. Lawrence for example was pacing for similar numbers to Dumais after being traded to Halifax and led the team in playoffs scoring.
Joshua Roy had a pretty dominant Q season as well in his D+1, and spent his entire D+2 working on making his game more translatable to the NHL,, hurting his overall numbers but he looks ready to play in the NHL now. He'll still probably start in the AHL, but don't be surprised to see him get games later in the year. Many Habs fans thought he had nothing to prove in the Q and Roy proved them wrong. It is about mindset. Drouin thought he had nothing to learn in the CHL and never progressed as a result.
Dumais still has a lot of things to work on to be an NHLer that can be worked on in the CHL. He's not a Patrick Kane-level talent, so he's not going to get by with Patrick Kane-level defensive awareness. His numbers look more impressive because he never made Team Canada, most top players don't play 64 games in a season. Drouin paced for more points than Dumais in his draft year and his D+1 yet wasn't given exemption status.
Anyways, Dumais won't be hurt development-wise from going to the Q again, he'll be hurt if he thinks he has nothing to work on there. I'm sure he's aware of that.