For the second time you treat me like an idiot and assume that I'm missing the obvious and excluding the income aspect. I'm not. Let me come out and state it triple explicitly so it's clear to you:
I see the income aspect. I realize that a loss of income is costly. I realize that income performance is correlated to the industry it's in and hence correlated with stocks in the same industry. That's the whole basis for the discussion and I see that. Jesus!
I see that, and I still disagree.
This is where both approaches can be right and this is the whole point I have been trying to make. For the sake of argument, suppose you have a trillion dollars in the bank and therefore infinite risk tolerance. Suppose also that your income is 100k. At this point, temporary loss of your income, which can also be reduced by unemployment pay, is wholly inconsequential compared to maximizing your gains in the stock market. If the tech sector pays back at 20% y/y (average) and the next best sector only pays back 5%, absolutely it's better to be in the tech sector even though your income comes from that. My example is unrealistic, but it serves a point: mathematically, it works. So the question is then how much risk (i.e. volatility) can you take in pursuit of maximizing returns? If you have a lot of assets, plenty. If you have no assets and therefore no risk tolerance, obviously diversifying your income from your investments is a good idea. But crucially, whether or not same-sector income/investment is a good idea all depends on how much risk (i.e. volatility) you can take.
Would you suggest Steve Jobs shouldn't have held Apple stock? I think it worked out pretty well for him.