Many people are frightened of drugs causing cancer, but there is a higher risk of cancer from immunosuppressants in those who have to take them to avoid organ transplant rejection as they have to take much higher doses. The fact is that if they didn't have the transplant then they would be dead - the immunosuppressants are just a necessity of that treatment and if they don't take them they could reject the organ and die too.
For many people with IBD the end outcome could be the same, but we can take lower doses of the drug and so the risk is lower. Having watched two people go through the absolute worst that this disease can torture them with, I know that I would rather keep taking the tablets rather than risk their fate.
I understand how you feel, and it is only by following this forum and trying to think rationally about these issues when writing posts that I keep my own mind in check about these things. I would love to come off Azathioprine sometime as the thought of taking it for the rest of my life is not a good one, but I'm not sure if I dare risk the potential consequences of that. I'm sure you will find others on here who have stopped medication without any problems, but many also relapse too and end up back at square one.
Please also remember that chronic inflammation, even if it is sub-clinical and doesn't cause obvious symptoms, can be a causal factor in bowel cancer.
We're damned if we do and damned if we don't with this disease!