and has this been more correct than using historical data?
And the fundamentals and valuations are subjective - people value different aspects of a company differently. Who's to say your valuation is correct? This is the reason behind volatility.
But if you're buying a broad market cap-weighted index, the behaviour of the price is less affected by individual valuations and more affected by macro-economic factors. And historically, a broad market cap-weighted index has had positive returns. This cannot be said about individual companie's performance.
People have been saying "the markets are fundamentally changed and investing is dumb" for ages. Japan has been the single example people point to over and over and over. Meanwhile, other people are looking at 400% returns over several decades.
it's the best you can do. As the saying goes, past performance doesn't indicate future performance. However, it's the best guess you can make.