I've read a lot of critical reviews and comments on the show and the lack of Madmartigan barely even gets mentioned. For example, the article that I linked above lists some of the common complaints and he's not even mentioned. I'm a fan of the movie, myself, and no Madmartigan doesn't even register on my list of things wrong with the show. I think that most fans agree that the character of Willow is what makes the franchise and would've been on board even if he were the only one returning, so long as the series felt faithful.
I was a kid when it came out and Willow was just this boring guy that waved a stick around and wasn’t very exciting.
He didn’t exactly move well or do exciting things, he wasn’t very funny or witty either.
He was basically a clumsy sorcerer’s apprentice that didn’t do anything particularly cool.
My bro, friends and I were big Madmartigan fans. He was the sword fighter, the guy with charisma, the guy that got the girl, etc.
We wanted to be him, not boring Willow.
I don’t see how you can build a series around Willow himself, and apparently you can’t.
I suspect if Val Kilmer was in good health ala Tom Cruise, and he reprised his role, it would have been an entirely different show.
“We can’t get Val… but how about… Christian Slater!”
Someone actually got paid to approve that idea.