True enough, but my understanding is that ketones are a alternate pathway for fueling a body that is under stress and can't use the preferred glucose pathway. Living under stress and existing by compensatory means is, as a rule, not a good way to go.

Not saying pure sugar is good. It needs to be accompanied by other nutrients to allow the body to properly make use of it. Of course, as you say, cancer cells can take advantage of it too, so there is that.