Hi Cargoshot, i've never heard root canals causing cancer, but i've seen very unhealthy and discolored gum tissues as a result of bad alloys used which in itself is as result of bad dentistry.
If you ask me whether or not extracting a molar without replacing it will cause problems, i would say very likely. A non existent tooth will cause its adjacent teeth to move and rotate in time and its opposing tooth to erupt gradually to the point that it may come in contact with opposing gum tissue, which may cause problems such as bite change and other. If this happens you may need to extract or crown the opposing teeth in time.
That said, some people may actually benefit from tooth extraction because their jaw is too small for their teeth which cause a lot of problems by teeth pressing against each other and causing similar problems i mentioned above, so extracting one or two teeth will actually benefit such people.