There must be some information content left in stock price time series data, as evidenced e.g. by the price Momentum factor [1], which was been replicated in a number of studies (e.g. [2]), observable over the last couple of decades.

[1] https://en.wikipedia.org/wiki/Momentum_investing

[2] https://papers.ssrn.com/sol3/papers.cfm?abstract_id=2961979