>> Cancer cells are 'immortal', no? <<

Being that they are mutuations which can take many forms, the answer is probably "it depends".

It is speculated by some that aging is part of a defense against cancer. The metabolism slows down over time. Slower metabolism reduces the chances of cancer.

I suspect that if they can stop the aging process that cancer will be even more of a problem. Cell replication boos boos build up over time as the natural course of the body. Keeping the metabolism at full force will probably make the problem even worse.

Screwing with Mother Nature is not easy.