Mordhau is fun and can be quite deep when it comes to duels but I Mordhau for different reasons compared to For Honor. Different mechanics and gameplay. For me it's easier to have fun in For Honor then it is in Mordhau, but when Mordhau gets fun, it gets really fun. The main difference is that in Mordhau every mistake can be fatal, but not just mistakes, you can also be one-shot killed from behind, never having realized what happened, which means it's easier to get frustrated. Mordhau punishes hard.
For Honor is more forgiving in that aspect, more like a party game if you want.
While both offer fantasy type combat Mordhau is a completely different experience.
It's not like Mordhau is a straight up better version of for honor, it's more like a better version of games like chivalry.
That being said, I don't believe Mordhau is a threat to for honor for the time being. It has far too many nuances to the combat for the average FH complainer to transition into and have much more fun with. The only way I see Mordhau posing a threat to the team and forcing a better bottom line for the game is if Mordhau ever makes it to console. FH's pc playerbase is small and thus Mordhau is going to have little to no impact on it.