Chemo is already the cure for many. It's a short round of very expensive targeted drugs which stop eventually. If that were true they'd just pretend chemo is forever and shift to a placebo version after they proscribed course.

Pharma aren't the only agents involved, academic researchers are a huge part of the system. They discover the cure then pharma pays for the expensive certification. Academics often do it for the science and maybe prestige. They're not likely to be muzzled. Governments have tried for things like climate but scientists will risk their jobs to speak. You can't buy off every academic.

The other flaw is, if they keep their 'cure' a secret they risk someone else discovering and patenting it. Then they lose out on any profit.