Let's not confuse singular risk and cumulative risk here chaps / chappesses. Using the Bomber Command example I'll try to clarify the situation:
1. Singular Risk: All other things being equal a crew had an equal chance of being shot down on their first op as on their last one if one discounts the benfits of greater experience etc.
2. Cumulative Risk: As the period of time that the risk is taking place and / or the number of risk events (ops) reduces, the chance of being shot down reduces proportionally as explained by Flyin 'Dutch'