The biggest number in the Trump Accounts rollout isn't the $1,000 government contribution. It is the nearly 70 million children now sitting inside a new investment system, one that initially directs contributions into a single State Street ETF. For State Street Corp (STT), that could turn a low-fee index fund into a remarkably long-duration customer-acquisition engine.

State Street Got the Default

The Treasury Department said automatic enrollment has created Trump Accounts for every eligible child under 18 with a valid Social Security number, taking the total to nearly 70 million. More than $4.5 billion has already been deposited, including government seed contributions, family money and philanthropic gifts.

But the money doesn't simply sit in cash waiting for parents to decide what to do.

At launch, the State Street SPDR Portfolio S&P 500 ETF (SPYM) is the exclusive default investment. Treasury selected four additional low-cost index funds, including products from Vanguard and BlackRock, but those choices were not initially available for parents to select.

These include the iShares Core S&P 500 ETF (IVV), the Vanguard Morningstar Total Stock Market ETF (VTI), the State Street SPDR Portfolio S&P 1500 Composite Stock Market ETF (SPTM) and the iShares Core S&P Total U.S. Stock Market ETF (ITOT).

That distinction matters. Every dollar that flows into an account before an alternative investment is selected is directed into SPYM, which tracks the S&P 500.

State Street's investment-management arm calls SPYM the lowest-cost U.S.-listed S&P 500 ETF, with a 0.02% expense ratio. The fee is tiny. The potential scale isn't.

Small Fees, Very Long Horizon

At today's scale, the economics aren't transformational on their own. At a 0.02% fee, the $4.5 billion already deposited into Trump Accounts would generate about $900,000 a year for State Street.

But Trump Accounts are designed to stay invested for years. Contributions can continue until the child turns 18, and the account then transitions into a traditional IRA.

That makes the more interesting State Street opportunity less about immediate fee revenue and more about duration.

The program has already attracted billions in philanthropic and family contributions. Treasury says more than 70 companies have committed to making contributions for employees' children, while the new rules also allow certain corporate and philanthropic donors to contribute individual stocks.

If the accounts become a permanent savings habit, State Street has effectively been handed exposure to a generation of investors at the moment their investment relationship begins.

The S&P 500 Is the Bigger Beneficiary

There is another layer investors shouldn't miss.

SPYM owns the largest U.S. companies through the S&P 500. So as money enters the accounts, the beneficiaries aren't just State Street and its ETF. The capital ultimately flows into the same mega-cap companies already dominating American equity indexes.

That means Nvidia Corp (NVDA), Microsoft Corp (MSFT), Apple Inc (AAPL), Amazon.com Inc (AMZN) and other S&P 500 giants could gain another source of long-duration demand as these accounts accumulate assets.

The immediate numbers are still too small to move those stocks. The more important question is whether Trump Accounts become a durable savings infrastructure rather than a one-time government program.

For State Street, meanwhile, the investment takeaway is unusually simple: investors should watch assets flowing into Trump Accounts, the pace of additional contributions and whether SPYM retains its position as the dominant default.

The headline is 70 million children. The more interesting market story may be the financial infrastructure being built underneath them.