The brain cannot function outside the body. The brain needs your bone marrow to make red and white blood cells. The kidneys and the liver to filter and break down metabolic waste. Various other hormonal systems that affect how the brain works (c.f. the HN favorite "gut-brain axis"). A brain separated from the body could survive for a few weeks, but long term it would certainly die from neuron loss (i.e. dementia).

That's on the same level as terraforming Mars to escape climate problems on Earth.

Removing cancer from a body is tremendously simpler than making a new body.