This mostly can't explain the fact that mortality is also rising in under 50s. It is true mortality is rising less than incident, and that a small proportion instances of mortality could be deaths related to reasonable risks taken on from treatment side effects (to make up numbers, it makes sense to take a 5% chance of dying from treatment this year over a 80% chance of dying from cancer in 5 years), but this is probably not the whole effect. Something is causing more CRC in people under 50.