Mage the threat range is because he's allowed to advance and charge, out of a Skyweaver that is also advancing and charging. You can kill him with a single unit of Guardsmen.
If you think a Harlequin Troupe Master disembarking and charging out of a vehicle is cheating, I think you gotta re calibrate your settings. That makes total sense within the lore and in game turns, its a move where you are giving up Warlord (and critical character) to take out another character or a small squad.
Yes jumping out a skimmer and fly kicking something in the face is very Harlie lore wise, but not over that distance. Most lore is effective written from a PvE perspective and they still don't go
that bonkers (most of the time lol). (Player verse Environment)
While there is no official time frame per phase, the game mechanic does have a time component. You can move forward with caution, or you can go full tilt, this reduces the amount of time you have to aim. Infantry carrying heavy weapons used to move or fire, due to how long it would take to set it up again, and so on. How far something moves is related to its potential speed, you can't have speed with out time. that' 50 inch coverage is a significant distance...
In a gaming system where it is PvP rather than PvE, the mechanics need to be a bit more like tic-tac-toe, in that for every move there is a a counter move. For example Feel no pain in previous edition could be bypassed with power weapons or double T, strength weapons, because feeling no pain is not really that useful when you've been chopped in half or taken an anti-tank rocket to the face, mind you that is also why high S weapons could out right kill multi wound models
This also meant every army could be built to have a counter to any units that had it, you then had a dance of one side trying to deploy the beef cake units away from the counter, or trying to take out the count before it could hit them. There was a reasonable chance for either side to deal with it.
Now you could counter a 50inch charge by dropping a character in the middle of a unit, but how easy is it to remove any non-horde unit, even 10 primaris marines isn't that difficult to remove, or to make a big enough dent to create an opening.. especially if your using a soup.. so if you want to protect your self from that move you basically have to castle up around your character... You really shouldn't be able to cover that much ground on a battlefield unhindered.
Games ultimately need to have a sense of fair play. While some people will get salty because they are snow flakes, some actually get salty because something happens that breaks that sense of fair play. The more that sense of fair play is broken, the less they are willing to continue playing. Ultimately I would feel rather crappy about pulling this sort of move on someone else, let alone if it was pulled on me. So, to me, this like a lot of other things in 8th, breaks that sense of fair play. 8th is also the first edition where I've heard people apologizing for a legit move before they do it, I guess that is both positive and negative in that its good people bad rules lol.