Yeah, that's because bitcoin is the rising tide that lifts all shitty ships. While your statement generally holds true, I think you meant to post it in another section and in another thread... unless you are fond of verbal abuse, that is.
No, I meant to post it in this thread, despite the customary "verbal abuse" as you say, because it demonstrates the principle that the "shitty ships" and bitcoin relationship is symbiotic as I've been pointing out for a few years now. The "shitty ships" buttress bitcoin's rise like the legs of the Eiffel tower so it can go higher. It would never have reached this level without the emergence of the altcoin market.
At a mature valuation, BTC dominance should be between 10% and 20%.
Bitcoin is the least risky investment in a high risk investment category. Its health does not depend on altcoins, but rather the health of altcoins depend on it. When bitcoin rises, it is considered to be an indicator that investor appetite for risk is on the increase, and that trickles into altcoins, which are the highest risk investments. When bitcoin falls, the converse is true, and altcoins sink accordingly.
If BTC dominance falls to 10-20%, it will mean that cryptocurrency as a whole is flourishing and has found niches among the general public. However, if BTC continues to rise and its dominance rate remains roughly the same, it will mean the general public is rejecting altcoins in favor of bitcoin, which was the case through the first couple months of the year. Either way, the price of BTC is not dependent on altcoins. It definitely has a unilateral, causative effect on altcoins.