This is incorrect, especially with ETFs. With an ETF, the seller/buyer (the individual, not the fund) pays all of the transaction costs. So buying and holding an ETF doesn't expose you to the problems which the behavior of panicking investors^. The same is not always true for mutual funds.
caveat: Vanguard index funds may be special and this doesn't apply to the same extent because the ETFs are a dual share class of the larger mutual fund.
But you are still exposed to the market, which can decline significantly. People will withdraw money from their passively managed funds when the market starts tanking. We have no idea what will happen in the next 'fear trade' when everyone starts dumping shares.
Stocks can go crazy and this could create a death spiral on the ETFs, because an ever more frequent decline can lead to significantly more volatility in small and midcaps, and more people withdrawing funds from their ETFs, which in turn will cause more liquidations.
All reasoning (to some extent) is lost as to which assets to liquidate. Actively managed funds will generally liquidate assets that don't cause too much swings in share prices in a downturn. Liquidating $50MM worth of Google won't make a big difference, but liquidating $50MM worth of SmallCap generally will put significant pressure on the share price. On the flipside: imo it does create a significant buying opportunity when it comes to small cap stocks.
Tl;dr: in my view, the more money that ends up being managed passively, the easier it will become to beat the market as an active investor.
There are two concerns: transaction costs (tracking error relative to index return) and index return.
Those who believe in generally efficient markets want to capture the market return, however volatile, with as little tracking error as possible. Since the ETF holders don't actually sell any stocks when the prices decline, their returns are temporarily depressed. If and when prices recover so too will their value.
There is nothing unique about ETFs in this respect. Your critique is more about index funds in general, not ETFs specifically. ETFs practically differ from mutual funds only in things like transaction costs.
re your tl;dr: Sharpe's theorem shows that active investors will underperform passive investors after costs. Notice that this does not depend on the number of passive investors (and certainly at this point we're nowhere near any of the percentages of passively managed assets which people say may be worrying).
Overall, I do not think your critique is well-informed by the facts.
To each their own investment style (and there are many), but markets (in my view, and in the view of many others) are not efficient. (More than) half of what determines the stock price is psychology and herd mentality. It's not just numbers, and more an art than it is a science. Larger cap stocks however are generally priced more correctly than small- or mid cap stocks.
Second to that is that your returns will also depend on the risk you are willing to take. Investing through index funds and ETFs will correlate with a certain alpha, but that doesn't mean returns can't be higher if you are less diversified (and thus taking more risk).
Depending on the type of companies you are investing in, you might be comfortable taking a bigger risk with the goal of achieving a higher return.
Say you're working in technology and truly understand it, you can probably outperform the market significantly by investing in 3 to 5 technology stocks. Is it riskier? Yes. Is it worth it? Some people will say yes, others say no. And that is absolutely fine.
caveat: Vanguard index funds may be special and this doesn't apply to the same extent because the ETFs are a dual share class of the larger mutual fund.