When bitcoin becomes too dominant the altcoin market falls which is bad overall as good alts like ethereum, eos, will also crash. 50% is just about right, if it gets lower then it's use as digital gold might waiver
Bitcoin is already dominant and even without looking at the figure, we will feel that there's always significant effect on altcoins when bitcoin moves.
The market data alone is not a good basis, you have to look in different factors to analyze the market pretty well.
That means if the dominance of bitcoin decreases in the market, lets say, the dominance of bitcoin is only about 40% of the total marketcap in the market. then effect of bitcoin is certainly not as big as before right? Bitcoin price movements may not affect the price of an altcoin market too much..