It seems like the result of this will be that Players are constantly dodging, diving, twisting around, and so on. Which is fine.
It happens, but not all that often. The Characters always do something, is it always dodging? Not really. Dodging doesn't let you hit the thing. A 10+ means you do it. You've dodged. GM gets to make another move. What do you do now?
Question: In your initial set up, the players see the monster and do something inconsequential. Then the monster attacks and they dodge. Why do that? "You see a group of monsters with big boulders in their hands. They howl and start coming within range. What do you do?" Here, you have made a soft move (Show signs of impending doom). The players better address the fact that armed monsters are within range, or just hit them and have them roll damage. That's your Deal Damage move. There's no dodging. Just damage.
But there is not always a cliff to a player's back to provide a threat to rolling poorly.
That's true, but there's always a threat. That's the GM's job and superpower.
Imagine:
Player and Monster in an open field.
Monster swings at player.
Player says:
1. I use my shield to block it.
2. I duck.
3. I run backwards.
4. I dodge.
5. I meet his paw halfway with my axe.
What should be done in each of these situations?
My thoughts using your set up.
1. Awesome, you raise your shield and the mighty blow rains down on it. Roll Defy Danger with Strength as the force of the blow threatens to drive you to your knees (put them in a spot). Or roll Defy Danger with Dex or the monster grabs a hold of it and rips it from your grasp (take away their gear). Or, you can deflect the blow but it will sunder your shield (tell them the cost or consequences and ask).
2. Cool, you duck as the scything blow whips out. Defy Danger with Dex is obvious here. But there is also, "You feel the wind whip past as the creatures claws swipe over head. It lets out an unearthly shriek and you feel the full weight of it's massive body as it barrels into you. What do you do now?"
3. I've already given running backwards a go. Here's another version: "Cool, as you start running backwards, the beast turns from you and lunges toward your friend the Bard with a fiendish howl. What do you do?"
4. Yeah, see "I duck" above.
5. Hack and Slash seems right here to me.
A. All evasive manuevers are Dex Defy Danger.
You set up the danger as "you'll be hit if you don't move" so people tend to move. Set up different dangers.
"The beast grabs hold of your shield. What do you do?"
"You and the beast start sliding down the ravine. What do you do?"
"A passing Wyvern is attracted by your scuffle and swoops in at high speed. What do you do?"
"The beast lands on your back, what do you do?"
"The beast grabs you by the throat and starts to shake you like a rag doll. What do you do?"
B. The problem is how to resolve 7, 8, 9. For dodge, what is a 7?
What does the 7-9 result on a Defy Danger say? It says offer them a hard bargain, worse outcome or difficult choice. So, offer up. "You can totally dodge that spear but you're going to be totally off balance." "You can totally dodge that spear but it's going to knock the sword from your hand and into the underbrush."
C. The problem is also what to do on a 10. Do they perfectly dodge? Great. Now what?
Make a move.
D. Shields and dodging are treated differently. Shields are part of armor. But dodging is an action. This asymmetry has potential to create problems.
Raising your shield can be treated as part of the fiction. It's fictional positioning for not getting hit, or defending a location or a person. There's no difference in the fiction. Mechanically, if you get hit you have +1 armor for your shield, unless something in the fiction has rendered your shield irrelevant. But I don't see a big problem. If they raise their shield to meet a blow, decide if a move has been triggered and go with it. And whenever they suffer damage that is reduced by armor, reduce it by 1 for the shield.
Bottom Line: At my session, my players never got hurt almost. THey were always twisting, dodging, etc. And I respect their descriptions and let them roll. I say: X swings. They say: I duck. And there must be some meaningful way of adjudicating this.
Don't swing. Make moves. Separate them. Put them in a spot. Take away there gear. Tell them the consequences and ask. Deal Damage.
I'm open to changing my mindset, but the example below did not help me (often there are not cliffs around).
So, use what is around.