I envision a Juggernaut as someone who can stick to a target provided they can get to it.

Udyr, Olaf, Trundle. They're juggernauts. Champions with Slows and Speed Boosts to keep in range, but strictly no Dashes or Blinks to get in range in the first place. Darius USED to be a juggernaut if you got TriForce Hydra Mercs and then Tank, Garen was more bruiser-ey (I just built and played him like a Riven most of the time), Skarner is and was a Juggernaut by my definition.

Mordekaiser shouldn't have been bundled with these guys at all. He's always seemed to be more like Rumble (to me at least) in terms of his Passive, Q, W and E. His R simply had a different purpose to Rumble where Rumble's is 'hit everyone you can, for as long as you can' and Mordekaiser's is 'hit the priority target and make sure it dies'.

I played Mordekaiser for a week probably close to a season ago during the height of Renekton, Shyv, Mundo, Ryze etc and never had an issue. Purely learning him out of curiosity. I got a Penta and 2 Quadras in that time frame.

Morde needed some love, not a knee-jerk rework. Movement speed on his W would have been a nice change if Riot didn't gimp his Ratios in the process. Then all it would have taken for him to be viable and strong would have been a Furor/Rage-esq MS Boost on his Q, or Movement Speed tied to his Passive shield.