Asking from a position of inexperience:
Why is it so hard to improve skating?
Does Mailloux have fundamental limitations like a lack of cartilage or always skipping leg day?
In my experience, some improvement is possible: a poor skater can become a less poor skater and a good skater can become a bit better skater. But except in the rare occasion, by the age of 20, what you see is largely what you get.
Suzuki is a good example of a player improving his skating after junior. Always, a skillful player, Suzuki lacked high end skating speed. That limitation was shown in the WJC, where Suzuki, facing elite talent, was largely ineffectual. His skating deficiency was why Vegas was willing to trade him away. Since coming to Montreal, Suzuki , while not an elite skater, was able to gain that extra step that made him a budding star in the NHL.
I have followed Mailloux's development/progression closely since he was drafted in 2021. To this writer, Mailloux lacks first step quickness and acceleration that would allow him to avoid forechecking pressure and to effectively transport the puck out of the defensive zone. It's a flaw that I don't see is fixable. All you had to do is watch him last night as the team was under pressure in the defensive zone and Mailloux (and others) were not able to cope and be effective against a heavy sustained forecheck. Mailloux is not an NHL level defenceman as he lacks the necessary skating ability. Just compare him to Luke Hughes who, with a couple of strides, was able to shed forechecking pressure and the puck was exiting the Devils zone.
With a back-end comprised of Savard, Xhekaj, Mailloux and, to a lesser degree, Hutson, we were caught slow against a speedy Devils team. And that overall lack of mobility will be fatal to any successful re-build unless this shortcoming is addressed.
Mailloux has excellent offensive talent. He sees the ice well, he has elite passing ability and a pro level shot. He's tall, has good reach and a nose for the net. Once he is in motion, Mailloux has NHL skating. Mailloux's future with the Canadiens is as a power forward, a conversion that should be easier to effect than most think based on his skill set.