If you mean VOO or even VTI, no, the S&P500 will certainly not only creep up in value. In 2007-2008 it went down significantly and it will do so again, maybe soon, since it looks a lot like a 1999 dotcom era to me, with AI and the Magnificent 7 top fanboy stocks of today.
https://en.m.wikipedia.org/wiki/2007%E2%80%932008_financial_crisis
https://en.m.wikipedia.org/wiki/Dot-com_bubble
That is why diversifying asset classes and geographies globally is important to me. My US equity is only 25%, the rest in cash, Bonds, Canada, Europe, Far East and Emerging Markets.
https://www.finiki.org/wiki/Asset_class
Through equity ETFs you are buying partial ownership in companies and their dividends, instead of buying the shit they sell.
Through Fixed Income or Bond ETFs you are loaning the government or Corporation your capital in exchange for interest payments and a promise to return your capital.
Through cash or GIC, you are guaranteeing short term funds are available when you need them (1-5 years out).
Gold, Silver, Real Estate, Bitcoin, Beenie Babies can all add to your diversification. The point is, don’t buy only Beenie Babies, they’ll never beat inflation.