Another common misconception in crypto community is measuring value of altcoins in USD. You can often find statements like "this ICO raised $20,000,000" or "that altcoin increased in [USD] price by 20%".
People ignore the fact that altcoins are pegged to Bitcoin, most of them can only be traded for it on exchanges, and even when there is an option to trade altcoins for fiat, most traders still chose the BTC pair
I'd rather say that your claim is the real misconception
Litecoin had been making local highs in BTC which is the opposite to your claim that "when Bitcoin is falling, altcoins usually don't change their BTC price, or even fall". Obviously, this is no longer the case. Major altcoins are pretty solid now and in no way are dependent on Bitcoin.
One anecdote doesn't prove anything (in that case LTC got pumped because it had SegWit, while Bitcoin miners were signalling), there is a strong correlation between BTC and altcoin prices.
Marketcap might have little or no significance among currencies in cryptocurrency market. But we can't neglect the relevance of marketcap when we compare two different commodities like gold and Bitcoin. Marketcap are very important to know about the acceptance and popularity of various commodities and for knowing the future potential.
It's a bit better in Bitcoin's case, because it has been circulating for so long, and has very high volumes now, and is traded against USD.
Its really hard to just conclude that market capitalisation is just irrelevant because of the issue about BCH because there is a basis for everything and BCH won't be any difference considering the fact that by default the BCH is created in relation to the availabe BTC out there and with one exchange having a price, its enough to calculate the amount of capitalization without any difficulty.
Marketcap is misguiding because you can create millions of coins, sell a few coins for some hundreds of USD and claim that there is "multi-billion network", "third most valued cryptocurrency", etc. This is just a method to trick noobs into buying shitcoins.