"You can't exist on two million these days. You can rub along on five million but two million just goes nowhere".
Obviously, it depends on your age and lifestyle, but it also depends massively on the cost of your ex wife / kids. If you keep your trousers zipped up, and avoid marriage, you can live (and fly, at a low PPL-type level of capability) on suprisingly little money. But while marriage is entirely avoidable, keeping one's trousers zipped up makes for a very dull life. PROB99, the biggest financial hit you will ever take is a terminated marriage, and PROB50 it will happen to you. And you cannot plan for it, short of concealing assets.
Investment returns are not so relevant. The historically higher returns were also matched by a higher inflation.