The why is the urgency to free up cap space to reload/improve right away. Ovie has 3 seasons left on his deal and at his age, playoffs next year will be the expectation...
I think it's clear the Caps will be aggressive this summer & moving Mantha will likely be a priority.
If they can do it without taking any contract back, I'm sure they would... But how many teams will without demanding a 1st or 2nd, or sending some other cap dump their way? They'll need to keep those assets if they want to make actual roster upgrades.
Armia, for his warts, is a better alternative to most 4th line options and can play decent 3rd line role. He's maybe 1.4M overpaid for what he brings but that amount will be less of a factor as the cap rises the following year...
For next season, Armia + 2.3M$ in cap space > Mantha @ 5.75M cap hit for a team in that kind of situation, imo
Trading Armia for a higher AAV / shorter term player would indeed be the optimal (and likely only?) way to shed his contract this summer without retaining salary and/or adding a significant incentive.
In that regard, Armia for Mantha as the main assets exchanged in a trade like you suggest could be a worthy gamble for both sides in my opinion; the Habs could of course afford to sweeten the deal with a mid-round pick or B prospect - nothing major.
At worst, the Habs still gain +3.4M cap flexibility 2024 onwards, when it will start to really matter.
In any event, with about only 20M AAV devoted to 8 defensemen and 2 goalies next season, the Habs can definitely afford to pull the trigger on 1-2 « Monahan contracts » on forwards. But I guess it will depend if they can first shed Armia and/or Hoffman from the roster to make room for such trades.