1) The probability of a H or T of any next coin toss (fair coin) is always 0.5 because coin tosses are independent of each other.
2) Stock markets, or for that matter any asset, are an entirely different game. First of all the expectancy is not 0.5 of, for example, experiencing an up or down day tomorrow in a stock simply because the distribution is different and because stock prices exhibit a drift component. Additionally, financial asset returns and especially their volatility exhibit co-integration properties of varying degrees. Thus, it does not pay to compare coin toss expectancy with asset return expectancy.
3) Changes in stock index levels as well as individual stock prices are the strict result of supply-demand imbalances and such imbalances are partly a function of varying and shifting sentiment on the macro side, industry sector side, and individual company side. Sometimes stock price imbalances can carry on for a prolonged period, at other times the market exhibits a strong drive to move the price back into what the market views as fair value regimes.
Such switches from momentum back to "mean reversion" and again back to momentum are what experienced traders are the most concerned with and attempt to assess in order to maximize the expectancy of their placed bets. I have not heard of long-term successful "market operators" to generate profits off the back of predicting what the next tick/bar/day is gonna be purely as a function of the up/down pattern of past returns. What I observed on the other hand makes a successful trader is the ability to assess as early as possible shifts in co-integration patterns after they occurred.
Just my 2 cents.