It’s absolutely not inherently wrong or implausible to assume that the constant and rather direct exposure over decades causes cancer.
Old timey radio operators definitely died earlier. They had much higher cancer rates. Granted, completely different levels of radiation, but radiation damage is stochastic. If there is an effect at all, it will cause thousands of new cases even low doses simply because we have like 7 billion phone users.
Doing proper studies on that is hard, but absolutely necessary.