Yes Mailloux has lots of great tools but also lots of negatives. You are right that in 5 years, we can look back at a trade like this and say woops that was a bad trade. But the opposite is also true.
Imo, Mailloux will be a number 4D if everything goes well but most likely end up as a very good 5/6.
There are two other aspects that make me think we could use Mailloux to upgrade our D in a good way :
1-Timing/experience : We cannot enter 2025-2026 season with Guhle-Reinbacher-Hutson-Mailloux as our top 4. That wouldn't work. Reinbacher is not ready for top pair duty and Mailloux is not ready for top 4 duty either. We definately need a bit more experience on that top 4 for at least a couple a year to let Reinbacher and Mailloux develop in what could become their respective ceiling. Maybe Savard is extended to give that veteran presence but even though i like him very much, we can't be succesful with Savard playing top 4 minutes for another 2 years after this one.
2-Better defensive D-squad. The way i see it, looking at the type of young Dmen we've got, we will need more defensive stability on the top 4. Hutson and Mailloux look to be defensive liabilities going forward and even if we extend the evaluation through the bottom pair, Xhekaj and Barron are not exactly defensive studs, to be polite.
That's horrible D to be honest.