In general a if a manager is changing formation in a game it’s because he’s got it wrong at the outset .
Exceptions could be dealing injuries , or packing the defence to defend a lead.
It is more commonly seen when chasing the game.
This can sometimes be the case, but it's a simplistic, glass half empty way of looking at it. For instance, you could plan your tactics based on an assumed approach which doesn't pan out, so the change is made, not because the tactic doesn't work, but that the tactic chosen is unsuitable for the other teams approach on the day. It could also be a case of noticing a weakness in real time and having the nous to exploit it. Your assumption is based on having to pick 1 single tactic and sticking to it regardless of what unfolds.
Changing a formation multiple times during a game requires a team containing a high proportion of intelligent players and those players come at premium and mainly congregate at the top clubs.
This is another assumption, that organisation and tactics are limited to the best players. When in fact, sometimes its these very attributes that help overcome superior players/teams when there is a mismatch in quality.
So most importantly, it requires a manager who is tactically astute and a good communicator, able to get his players to do as he pleases, when he pleases.
4-4-2 has enjoyed longevity because it is simple and easily understood . It is the go to formation for managers inheriting relegation haunted clubs for that reason.
It is however mostly a reactive formation and often means surrendering control of the game against more proficient and aggressive attacking teams.
442 has enjoyed longevity for the very reasons Carlo has himself explained several times. It is literally THE most balanced structure in terms of pitch coverage and the balance and symmetry of it is the best basic shape for a team to fall back into when not on the ball. It's simplest form is THE simplest form. This doesn't mean, with the right group of players, and the right understanding about where and when they should be, that you couldn't get a 121321 formation to work, but the more complex the formation, the more difficult it is to communicate that to 10 individuals.
The old Shankly quote, "Football is a simple game, complicated by idiots" is at times quite apt. But sometimes, the subtleties of the best plans can appear simple to people who don't know.
The best make everything look easy. And Carlo is definitely of the best. And if he thinks 442 is good for him, and he recruits based on that opinion, then none of us are really qualified to argue.