Siegel is a perma bull and the other guy isn't You can't side with someone that only bets on one side because eventually they lose
Basically Siegel is saying "no matter what valuations are, this time it's different due to low interest rates." In the long run, "this time it's different" never works. I remember it being used to justify the way overvalued 90s tech boom because we were now in "a new era."
There is no such thing as 'over/under valued' or 'over bought/sold'. Thinking in those terms sets up a traders & investors for endless losses. 'Fundamentals might be good for the first third or first 50 or 60 percent of a move, but the last third of a great bull market is typically a blow-off, whereas the mania runs wild and prices go parabolic... There is no training, classroom or otherwise, that can prepare for trading the last third of a move, whether it's the end of a bull market or the end of a bear market.' Paul Tudor Jones
Siegel assumes interest rates are static and never change. Just like he thinks that historic real returns were 6% for US stocks so it will remain that way until infinity. He also thinks declining productivity and slowing GDP growth compared to post WW2 USA does not matter. He's delusional.
Siegel projects 5-6% nominal returns going forward, at least he did in an article about a modified CAPE ratio. His modified CAPE makes a lot more sense than Shiller's
He was on CNBC in 2010 saying he expects 6% real, "just like equities have returned over the last 200 years. There is no reason think the next 200 years will be any different".
To his credit, if in 2010 you followed the recommendation in his book and bought the top stock on his list: Altria, you would have a compound return of 21% per year, better than Warren Buffett.
In the super long-term, valuation doesn't matter as much so I would say his comments aren't as crazy as they sound