Let me also confirm that I fully commit to low expense, broad index fund, passive investing. I fully embrace the Boglehead mindset. I find great comfort and ease in the content and wisdom of the posts I read here.
Based on our age (mid 30s) we should be - and we are - taking substantial appropriate risk in the market, to the tune of 90/10 per Vanguard TR funds (2045).
But in the back of my mind I can't help but feel continually uneasy about pumping (90/10) into equities right now. Every time I hear about yet another market metric record high I honestly feel a little disgusted. 2018 is the first year we are paying ourselves first to the maximum extent our budget allows. Looking at the below S&P chart straight off Vanguard, could there literally be a worse year in history to have begun investing heavily for retirement?

I found a recent thread very interesting (if not somewhat frustrating): "Anyone tired of the stock market going up?" It was pretty enlightening to see the numerous and various responses, seemingly based primarily on where a person is at in time in relationship to their retirement. I could mention a dozen fascinating responses but the below two are most relevant to my question:
"Who's doing all the buying, the pros or the performance/index chasers? If it's the latter, this will end very badly for them." - This resonates strongly w me and reinforces the concern I have about being one of the many who are pumping large %s of our income into TR funds right now (for the simple purpose of a stable retirement) .. but to who's benefit?
Moderator Mel Lindauer said the following in that thread: "Yes, younger folks making regular contributions to their 401k or similar retirement plans should pray for a prolonged down market. Remember, the only two prices that really matter are the price you pay the day you buy and the price you get on the day you sell. Everything in between is merely "noise. So continue to make your contributions when the market tanks, knowing that over the longer term, you're buying at "sale" prices." - I don't particularly want the market to make a sudden drop nor do I pray for a prolonged down market - that has significant impacts on every single working person's life. But this does reinforce my concern that our current aggressive AA may not be entirely appropriate. After all, where are these supposed "sale prices"?
My actionable question here is basically: given the aggressive AA recommended for our time line, is a ton of money really being left on the table if we temporarily moved to something like 80/20 or 75/25 in this unending, vertically-moving market? Based on some posts I've seen in multiple threads, this might even allow for additional ammunition for when the market at least isn't moving near vertically.
Am I engaging in blatant market timing with this type of thinking? Or is this at all reasonably strategic? I sense that some "find your comfortable AA and stick with it" type responses will come. This isn't really where I'm at. I'll circle back now to the Warren Buffet quote at top. Given our time line I absolutely want to be buying up as many broad index equity funds as possible. But how can that be reconciled with a market moving near vertically for so long, so "greedily"?