One observation regarding 2017: the USD dropped by quite a bit vs. most currencies. That gave a strong return boost for most here, which is partly 'funny money' on the world stage. I had a crazy year a bit before that when the EUR went down the tubes for a while, doesn't mean much.
Another observation: inflation. If it stays around 2% it actually justifies higher P/E multiples. As I wrote in another post recently: Earnings yield is an informative measure too (CAEP10, if you will). It's now around 3%.
With inflation at 1.5% or so you are still looking at 1.5% real returns, excluding any additional growth from reinvested earnings which are typically around 3% per annum. 3% real incidently is at the lower end of most SWR estimates, now you know why: that's the rate at which real earnings have been growing. It's remarkably stable.
So my personal view is that as long governments are able to keep inflation under control (say, below 5%), P/E multiples will never be in the single digits again. A tall order, but signs are that we have become better at it in most developed countries.
If you look at the past several severe drops, they were all happening when inflation went above the real earnings yield.
Whether or not to exclude 2008/09: it gives a bit of a difference, but bear in mind that a lot of the losses posted were an opportunity for companies to completely dump (and overdump) some of their skeletons in the closet. So you'd need to correct the other way as well.
Personally I use the CAPE-10, bond yields and inflation measures as yard sticks. That, coupled with the Graham wisdom: stay within bands. As in, ok to adjust your allocation, just be sure to stay e.g. between 30% and 70%. And have a decent chunk of cash to whether temporary downdrafts.