Valuation doesn't matter to stock market investors for very long periods of time so, with this particular momentum bet, valuation doesn't matter to me either - but I'm willing to dump it in a hot minute.
We hold a boatload of stocks with capital gains that have more than doubled, with some stocks up ten-fold in price since we bought them, but I'm not currently prepared to pay out a 17% tax plus another chunk to California.
If I sold all of our "over-valued" shares and paid the tax right now, it's not clear to me what I've invest that money in. For now we're better off to let them ride.
We've run out of other people's Social Security taxes needed to subsidize our low income tax rates.