Double and triple pendulums have been solved: https://www.youtube.com/watch?v=cyN-CRNrb3E
I have little doubt cancer will be someday as well. It might not -- probably won't -- happen in my lifetime. But if humanity hasn't collapsed, I find it very unlikely that we're still dealing with cancer in a thousand years.
Agreed that we don't yet know how to regulate the apoptosis, or understand how metastasis works. DNA Methylation, small RNA regulation, and purpose of non-coding RNA isn't very well understood yet, too.
> I believe it's much easier to simply figure out the right cocktail that would change cancerous cells into normal cells again, and then find the delivery mechanism for that.
I reckon much of this has to do with small RNA and miRNA based regulation of transcription. Essentially the specific portions of the RNA/DNA are "silenced" by several regulatory factors. If a growth regulator gene, say CDKNA is accidentally silenced then the cell would undergo an uncontrolled growth, _with_ mutations. This in turn will grow "diseased" cells, eventually causing cancer. Trouble is, we just do not understand this properly, yet.