Resistance is a neat thing.
I remember (and I'm sure someone will reply with a link to it, since I can't find it) reading a story about a fellow with a very seriously antibiotic resistant bacteria infecting his chest. It resisted everything they had. Researchers found a phage that would attack that specific bacteria- but alas, the bacteria became resistant to the phage as well!
Fortunately, the bacteria had limited dimensions in which to change. The changes needed to become resistant to the phage made it no longer resistant to at least one of the antibiotics. It could not have its cake and eat it to. With both attacks against the bacteria ongoing, it was defeated.
What I'm implying with all this is that yes, the cancer may become resistant to this therapy in question but in doing so it may be forced to become less dangerous, or less resistant to other forms of attack. It does not change merely from "not resistant" to "resistant" but from "form A, which is not resistant" to "form B, which is resistant", and "form B" may have a lot of other consequences.