Since cancers are immortal, as long as they are ‘fed’, could a bunch of controlled cancers be used to create all the organs and bones needed to support a human and prevent death by old-age? That way, the ‘person’ lives forever and doesn’t have to worry about cell apoptosis. Also, let’s assume, that since the cells already mutated into a ‘cancer’ that they can’t unmutate or mutate again to lose the ‘immortality’ status.
By definition, no
Let's look at the definition of cancer:
Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread.
Further down in the same article:
All tumor cells show the six hallmarks of cancer. These characteristics are required to produce a malignant tumor. They include:
- Cell growth and division absent the proper signals
- Continuous growth and division even given contrary signals
- Avoidance of programmed cell death
- Limitless number of cell divisions
- Promoting blood vessel construction
- Invasion of tissue and formation of metastases
Put simply, a cancer is by definition uncontrolled and invading other parts of the body. 5 out of the 6 characteristics of a cancer mean that a body made up of cancers would have each part of the body fighting with every other part and burning up resources unproductively while growing uncontrollably.
If you are going to come up with a new condition that just gives one attribute of cancer while ignoring all the other characteristics then it isn't cancer. It would be like holding up a front windscreen for a sedan with no other parts of the vehicle attached, calling it a "car" and praising its fuel efficiency.
If you want a body continue living whether or not it has signs of ageing then cancer is not the solution.
A cancer lacks the structure and function of the organ where it grows: a lung cancer doesn't increase the lung functional mass, a brain tumor doesn't increase the functional amount of brain and so on.
Just growing cancer everywhere in the body would slowly replace the functional body with a blob of unstructured cells whose only goal is to replicate, which cannot be a functional organism, since all vital functions have to be secured by some of those organs which have being turned into the blob.
Cancerous tissue can't replicate the functioning of a healthy organ. This is why people die from cancer. The rampant growth disrupts the body's ability to self regulate and continue to sustain life. When someone dies dies their tumors will quickly die like every other tissue in the body.
There are exceptions to this inevitable fate, ways to cheat the reaper his due. Clonaly transmissible cancers can spread from one individual to individual. Effectively becoming an unicellular, asexual, pathogen. Similarly immortal cell lines are manually propagated for research. Each cell identical to those originally harvested from some cancer patient.
Few would consider what survives a person. Fewer still would consider its existence proof that someone is still alive. Their body will undergo funeral rites. If aware of the cancer loved ones may find some solace, but they will still grieve their passing.