D&D 5E Can you use misty step to arrest a fall?

[Maxperson]

He is typically in the middle of combat ducking arrows, swords spells or whatever and fighting for his life. I would argue it is similar.

No. It's not similar at all. Combats generally aren't a Loony Tunes cartoon where there are hundreds of arrows zipping by. Plummeting uncontrolled to feet to your death is much more distracting.

 

[cmad1977]

If a player expends a resource I generally allow the action.

For example: I will let a player use an equal or higher level “water” spell to counter a “fire spell” and the like.

 

[Gammadoodler]

That can create some other interesting ideas to utilize...if everything falls instantly...If I create a hole through the planet to the other side and step through it, do I instantly get to the other side?

And other puzzling questions like that...

Keeping mind that D&D physics isn't real gravity.. gravity works through attraction between masses.

We fall "down" because the vast majority of the mass of the earth lies "below" us and it's mass is way larger than ours. This changes as you move through a planet. The closer to the center you get, the more of the planet's mass is distributed around you rather than below you.

Assuming a path through the center of the planet and uniform density, you would reach a point where your body would not have a "pull" in any particular direction. It would feel like weightlessness.

Assuming you continue to the other side, as you get closer to your exit point, more and more of the planet's mass would be "above" (or "behind" if you prefer). At the exit, all of the planet's mass would be, consistent with when you entered. With nothing to propel you out, you would "fall" back toward the center of the planet's mass.

Been a while since that particular physics class, but IIRC absent external forces, you'd essentially keep falling back and forth forever.

tl;dr
Falling through a gravity well does not work like you think it does.