Well first of all, it is a very naive analysis. It disregards the closing of loopholes which will likely occur. Trump wants the earnings currently being sheltered overseas to be repatriated, and there will be a stick as well as a carrot. So while the tax rate will decrease, taxable income will increase.
Secondly, raw P/E is a pretty useless valuation measure. You can use Buffett's measure - market cap/GDP or Hussman's - GVA/GDP if you want the best predictive value. Needless to say, both are at hair-raising extremes and are unaffected by tax rates. Earnings are subject to so much manipulation - I've sat in the end of quarter meetings myself - that they are largely meaningless in the short term. CAPE aka Shiller P/E is not too bad, but it uses a 10-year average as well as an inflation adjustment so would see little impact from tax changes.
If you are looking for Schadenfreude at the expense of those who care about valuation (they can be referred to as investors, for short), look elsewhere.
Edited by libertas, 22 October 2017 - 01:30 PM.