And important part of a functional immune system is checkpointing and surveillance of cells. Cells with errors in transcription in theory are flagged for destruction. Cell surface signaling is another way that the immune system jumps in to attack abnormal cells. This process is happening continuously, so in a technical sense if you wish to define cancer at the level of a single cell with proliferative potential, then sure, cancer is popping up all the time. As a result it is also why immunosuppressives carry warnings about increased risk of cancer, because they are downregulating a critical safeguard.
Cancer detection is still predominantly at the level of macroscopic disease, at which point it has far surpassed the point of failure on a cellular level. While a rise in cancer diagnoses are to be expected with improved screening, a hypothetical situation where cancers are coming and going does not occur at a clinically detectable level.