Death of X #1 spoilers

[ZNOP]

Dr. Hydro modified the Terrigen Mists so they wouldn't kill humans and would provoke always the same effect on them (turning them into amphibians), I think.

You have a good argument there... But, I just finished reading Marvel Two-In-One #72, and there was no mention of Dr. Hydro modifying the Terrigen Mist.

Anyways, the greatest retcon isn't the lethality of the Terrigen Mists, but the fact that they only work on Inhumans. For the longest time the gift of the Inhumans weren't their latent powers, which they shared with the rest of Humanity, but the fact that their enhanced biologies allowed them to survive Terrigenesis.

And, let's not forget that the "Anti-Terrigen" compound (see: Marvel Two-In-One #71) that Reed Richards and Nadar developed was a counter to Terrigen transformation which would make it a potential cure for for mutants.