Historically, one of the benefits of purchasing index mutual funds and ETFs has been the simple transaction cost savings of being able to buy into a diversified portfolio of securities in a single step (pooled) transaction, rather than absorb the transaction costs of buying a small quantity of each individual investment one at a time (not to mention the sheer trading complexity of trying to do so). Yet since fixed trading commissions were deregulated in 1975, transaction costs have faced an ongoing inexorable decline, as competition from discount brokerage firms, then online discount brokers, and more, drive the cost of implementing a trade closer and closer to zero (or at least, to the point of being truly negligible).
Yet at the point that transaction costs really do become negligible, an interesting question arises: why bother to buy index funds or ETFs at all, instead of simply using technology and near-zero trading costs to buy, own, and manage a portfolio of all the underlying individual investments directly. At face value, the investment characteristics may be the same either way, but for tax purposes there is a key distinction: owning each individual investment at such a granular level allows investors to be far more selective about proactively harvesting capital losses (or even gains in certain situations) or taking more tax-sensitive liquidations in retirement (and of course harvesting itself is easier in an environment where there are no trading costs to engage in such strategies in the first place)!
The end result is a kind of "Indexing 2.0" solution, where investors still hold the investments of an index fund, but without actually using a pooled index mutual fund or ETF itself, opting instead to own the underlying individual investment directly and take advantage of the associated tax benefits. And the concept isn't entirely new; in fact, Wealthfront offers a version of this exact service to replace the S&P 500 with the underlying investments against which proactive capital loss harvesting can be done on a daily basis. While this "Wealthfront 500" service still has a $500,000 minimum, it seems only a matter of time before ever-declining transaction costs make this feasible at a lower and lower price point... until eventually, there may still be value to indexing itself, but there will be little reason for index mutual funds and ETFs at all?