That'd be ridiculously riskier of a fumble compared to being prepared to hit the dirt or (in some codes) cry "down".

So then what's wrong with loss of down in any of those situations?

Truth be known, I'm not a fan of allowing them to "gain" back to the previous spot on any incomplete pass, but in consideration of the uncertainty of completing one, the rules makers long ago decided to award that compensation. Nor do I like the idea of timing's being different depending on whether the ball becomes dead by incomplete pass or by run in or out of bounds. Yet the adoption of both such considerations have led to the further complications we see re intentional grounding and the spike.

I would think "Loss of Down" should be automatic, after all the act of dumping a pass means the play the offense called failed, one way or the other, so why would even think of giving that down back. Rule wise, it is what it is, and that's the way it will bedealt with (until the rules makers decide to change it).

I believe, at the NFHS level, the Illegal pass is considered a running play, (NF: 10-3-1a) which eliminates the timing confusion.

The problem with allowed dumping (outside the tackle box) is that: 1. The offense chose which type play to call. 2. The defense has to respond to whatever is called. When the defense does it's job and prevents the offense from attempting a legal forward pass, why give the offense an unearned opportunity to negate their own failure and the accomplishment of the defense, by simply dumping the ball. The NF rule also provides for enforcing the penalty from the end of the run (with an illegal pass, the spot of the pass) whether it's beyond, or behind, the LOS.