I agree with some of the points but a note of caution. Stock markets today are significantly different than those of the past and the kind of low interest rate, low growth environment, we've been in over the last decade or so has no precedent.

"In the 10 year period from 2006 to 2015, the average return was a little lower than 8%"

From the peak of the '99 bubble till around today we have about 2% yearly return on the S&P 500 (excluding dividends). You would actually do much better if you were in bonds. Between '99 and '09 you would actually lose money. Two takeaways from these, one is that you can't just pick some period and build a theory over that, the second is that when you're invested in stocks there is a non-negligble probability of losing money over a 10 year horizon. The only reason stocks are so high these days is that their prices are supported by zero interest rates. That doesn't mean they can't go higher for various reasons but you need to be cautious. Everyone talks about buying stocks right about when things get frothy, not too many people post this sort of financial advice at the doom and gloom bottoms. Over long periods, dollar cost averaging, you'll do OK. Don't rush in at a top and obviously don't sell at a bottom, something a lot of people end up doing.

Privately held tech companies, especially startups, can actually have a better return vs. public companies. The problem is not the return, the problem is getting a strong, diversified portfolio. Unless you are a VC you can't really do that. In general small caps tend to outperform large caps and startups tend to outperform small caps, in aggregate, over long durations. It's really not about out-picking stocks, it's about being able to diversify.

There are a few factors affecting diversification. Different markets and asset classes tend not to be perfectly correlated. This means that some may be overvalued at the same time that some our undervalued. While it's not always easy to tell (sometimes it is easy, when no one wants to buy) I would think one should offset their weighting to areas they consider to have better value. As long as those areas are themselves well diversified (e.g. Europe or Emerging Markets) the long term risk you are taking is low. The other factor is that having multiple assets allows you to construct a better portfolio. This is known as the "Efficient Frontier". Assuming you have some information about how the different assets correlate with each other (which is a big assumption but still) you can combine those assets to create a higher returning portfolio with less risks.

Personally I'm invested in a mix of stocks (worldwide), fixed income, real estate (through funds) and bonds. I keep adding to this. I make some macro bets (e.g. I've been heavier Brazil, Greece, emerging markets, junk bonds ATM) through long term weighting of my portfolio and I keep an eye on those. Out of my current bets Greece hasn't worked out (yet) but the others have. YMMV. These are not the kind of bets where I can suffer heavy losses over extended periods (IMO) but there's certainly increased risk. I don't expect US stocks to have great returns over the next decade or so but I'm still in there with some portion of my investment. My horizon is 10-20 years and I very rarely sell anything (except the stock I get from work :). I try to buy when people are panicking but it's hard to find good panic these days ;)