I left a comment over at the site. In short, I also think the 5% assumption is ridiculous. I have perfect data of the date and amount of every retirement contribution I've made since I started in 1993. I used historical data to look up what my APY would be if I had bought an S&P-500 index fund for each of those dates/amounts. I also compared it with historical inflation records. As of today, it wouldn't be 5% after inflation - it would be 0.9% . And this is a good period - for the vast majority of that time, it would have been a negative APY, and that's over the last twenty years.
That number will be different for different dates/amounts from different people, but I can tell you that I have saved pretty consistently and aggressively over the last twenty years, and there's nothing out of the ordinary with my savings schedule - no huge gluts just before a crash or anything like that.
Not sure anyone is reading this thread anymore, but I have to partially retract. In my case, when doing the historical analysis, I thought I had taken dividends into account but I hadn't. So in my case my historical APY would have been about 2.75% (factoring in inflation), not 0.9% . That still shows that 5% isn't close to a safe assumption, but it's not as overwhelming as it was before.
That number will be different for different dates/amounts from different people, but I can tell you that I have saved pretty consistently and aggressively over the last twenty years, and there's nothing out of the ordinary with my savings schedule - no huge gluts just before a crash or anything like that.