I once had someone ask if even the worst writing could be saved by editing.
Here you have to define editing. Editing can save a work that has extreme technical issues…that is, the skill level of the writing is poor.
Editing cannot address storytelling aspects, the structural issues such as character development, plot milestones, beginning and ending, etc. These can only be addressed through revisions.
Nearly all works can be rescued either by editing, revising, or a combination of the two.
What sometimes cannot happen is the shift of a manuscript from one category to another. If someone brings me a genre work and wants to shift categories, a heavy revision including rewriting can achieve that goal.
For example, a romance writer wants the manuscript to become a mainstream drama or a mystery author wants the manuscript to become a commercial thriller. In each case, the changes must address both plot and writing style. This often results in such heavy revisions and fresh writing that the plot might be similar and the theme the same but the book is entirely new.
In some cases, even this extreme level of work won’t reach the goal. Most of the time that’s due to the plot being unable to carry the weight of a different category or simply being too thin to handle the more in-depth treatment required by a different category.
So: yes, very bad writing can be saved. But even very good writing can’t always become something it’s not.