I don't recall the last time a fight between champions lasted as long as 5 seconds. It's always a cluster of "wtf just happened?". Fights between champions don't last this long. Chasing an enemy down to kill them might last that long or longer, but that's not really a fight. That's just trying to mow down some straggler.
I can imagine how this ult is actually going to be used because 7 seconds in league time is twice the duration of eternity. Morde won't have to build AP or AS or Bruiser.
No, because he has 7 seconds to remove an enemy from the equation, all he has to do is survive them for however long it takes his team to take care of the rest. He's going to be built tank, a very tanky tank, because his kit allows it. It won't matter if the carry survives. All that matters is that Morde singled out a problem and neutralized it so that his team could take a turret or a dragon or a baron etc.
Everyone is crying about how Morde is going to have this power to obliterate some squishy, but why does he need to be bothered with that? He's not an assassin. That's not his job. He's a bruiser. All he has to do is survive and bruisers are good at keeping their health above half full in 1v1's.
His ultimate will prove boring, his ultimate will prove irritating, his ultimate will prove unhealthy for game play in a team based game, and Riot will likely realize that they made another mistake as players proceed to play Mordekaiser in a way they didn't intend.
Riot, you're not making a duelist. You've turned that asshole coach in professional ball games into a champion. The one that instructs his team to stall out the clock rather than play ball in a sportsmanlike manner.