Tax-managed index funds defer capital-gains distributions by, among other things, offsetting capital gains with losses and deviating from their underlying indexes to avoid having to sell appreciated assets. But if the funds are successful, it seems that the share of unrealized capital gains will grow indefinitely. Right now, the unrealized appreciation of Vanguard Tax-Managed Small-Cap Fund Admiral Shares is 28.4% of NAV. As long as the fund delivers decent returns over the long term, is there anything stopping this amount from ballooning to, say, 90% fifty years hence?
If this happens, won't new investors be scared away by the prospect of owing taxes on these gains? For example, a financial crisis or a superior new investment technology could lead investors to dump their shares of tax-managed index funds, triggering enormous capital-gains distributions. And if new investors are scared away, won't the fund be forced to sell its assets to cover redemptions (even if there is no disruptive event), leading to larger capital-gains distributions than in the past?
Finally, do ETFs avoid this problem (assuming it is a problem)?