Are you covered with an ETF savings plan?

Hello dear community, probably a stupid question, but if, according to an online calculator, I invest €400/month in an ETF pump for 40 years, I'll have several million by the time I'm 65. After that I can easily retire and chill out like a king. 4000€ living costs x 12 months x 20 years are only 960,000€. There's easily a million left over and if I die before then... my children will just get it and let it continue. Am I missing something in my youthful naivety? Can it be that simple?