I guess it is wise to sell when the market is greedy and buy when it is fearful, as long as you have the foundations in check.
I am more of a holder usually as well, unless I truly believe the price is so overvalued that it would take a lot of time before another full dip-rally cycle.
If you have been an investor (or startup employee) for less than 13 years, you may not have witnessed large paper losses that make a good argument against never selling.
More directly: We are in the longest bull market in history. It is not always like this.
I've learned that it makes sense to miss out on some upside to protect against the downside. This was a reversal of my previous beliefs, which matched the GGP's.
I make no predictions about the future here -- short term, or long term. Somehow we (the investor class) survived a year of enormous social disruption which should have decimated the market, at least. The popular wisdom says that the next year will bring (social) recovery and economic expansion. This might be true.
But I would argue against a blanket policy of never selling. I have ridden that train all the way to $0 (literally) more than once, and while I was not harmed by these decisions, they were objectively poor ones.
Everything you say is true, but right now there is no obvious way to protect against the downside except to sit in cash. If the market rallies for another 10 years we will all look rather foolish doing that.
Yields are falling across all asset classes, and correlations between them are growing.
The overriding feeling no matter how much analysis you do is that nobody has a bloody clue what is going on.
I once believed this to be a truth that I had learned.
Then time moved forward, and I learned that it was not a truth. :)