The stock market is relying more than ever on the world's hottest investment product

As one of the most important drivers of the eight-year equity bull market fades, another bullish catalyst looks primed to fill the void.

Exchange-traded funds will purchase $300 billion in equities this year, more than in 2015 and 2016 combined, according to Goldman Sachs. The firm raised its forecast after ETFs were responsible for $98 billion of stock buying in the first quarter alone.

It's just the latest in a flurry of signals that the $2.8 trillion ETF market is not only growing rapidly, but expanding its market influence as it gains in popularity.

Credit Suisse was forecasting in May that ETFs would see record annual inflows, while Moody's predicted in February that passive investments would make up 50% of the US stock market by 2024.

The shift toward ETF investment comes at an ideal time for market bulls. Corporate share repurchases, which have served as a crucial backbone for much of the eight-year bull market, were down 18% in the first quarter on a year-over-year basis, according to data compiled by S&P Dow Jones Indices.

Even then, buybacks were still the biggest driver of equity demand during the period, Goldman says.

But the firm acknowledged that it scaled back estimates for repurchases amid expectations that tax reform would be delayed. President Donald Trump's proposed repatriation tax holiday was expected to boost domestic cash holdings, but no reform of any kind has materialized.

The resulting gap in equity demand has been bridged not just by ETF enthusiasts in the US — foreign investors have also gotten in on the action. They bought $55 billion in US stocks during the first quarter following two straight years of negative net demand, according to Goldman data.

Those foreign traders may be further persuaded to sink cash into US ETFs if dollar strength slows, the firm says.

Still, amid all the pro-ETF arguments being espoused, Goldman urges caution for investors blindly pouring money into such funds. After all, experts across Wall Street are expecting the S&P 500 to finish the year basically unchanged from current levels. On average, they see the S&P 500 ending the year at 2,414 — less than 0.1% off last Friday's closing price — according to a Bloomberg survey of 19 strategists.