BCash cannot go up by its own. Its price is totally depended if Bitcoin is up. Their claims of BCash is going up when Bitcoin is going down is a pure price manipulation they cannot do it everytime when Bitcoin is down or they will lose a lot of money from buying BitcoinCash at loss.
Bitcoin price is going down day by day..
Is bitcoin cash have that much potential to overcome the bitcoin?
Yes, the price movements of Bitcoin Core pretty much dictates the direction of the market. It dominates the market - mostly because it managed to claim the name of the original Bitcoin. Moving forward - this is expected to change. E.g. given the current level of its dominance - around 37% - it shows a decline of more than 60% compared to the peak dominance it reached. It is just a matter of time before it will be overtaken by another crypto.
And yes, price manipulation certainly plays a role in the price movements of most cryptos, but there are other factors that are influencing prices as well. To claim that the price of BCH
"is totally depended if Bitcoin is up" and that
"claims of BCash is going up when Bitcoin is going down is a pure price manipulation" is a bit of a stretch. The extent to which price manipulation plays a role is anyone's guess, but to claim that it is 100% the result of price manipulation is far-fetched. E.g. BCH recently more than doubled in price while the same couldn't be said of BTC. The reason for this is partly because of the positive changes - perceived and real - coming for BCH on May 15th.
"they cannot do it everytime when Bitcoin is down or they will lose a lot of money from buying BitcoinCash at loss" - Yes, assuming one is holding BTC and wishes to switch to BCH, the best strategy is to make the switch from BTC to BCH while the BTC:BCH price ratio is at its highest. However, not all people who switch to BCH are doing so from a position holding BTC - so to claim that all will be losing money when they buy BCH when BTC is down is not entirely accurate. In addition, you will need to assume that they got in at the same price levels, are selling at the same price levels and are buying at the same price levels again - which is not the case.