Stakes here is really a shorthand, a statement of intent. There is normally still room for some interpretation of the roll by the GM depending on whether it was a normal failure or a crit, etc.
Note that as the example you provided was an attack roll, in D&D the stakes of this are normally crystal clear - roll the fixed calculation of damage. You're adding a descriptive flourish to that, and quite rightly so, but it's no different to what a GM in another game might add to a stakes-already-set dice result to give it a particular spin or sense of depth.
What you describe isn't unreasonable, and I might well do something similar (albeit in the open). I'd say you have softened/reinterpreted the crit result rather than fudged it away. The PC here is still badly hurt in a way that can't just be hitpointed back on, so you have preserved the poor swing in fortune that occurred.