Bitcoin market dominance have decreased significantly over the last few months this is due to new money flowing towards other alts like to ETH. However I don't think Bitcoin will ever loss its charm because it is still the most secure and strong crypto out there. Support for BTC is really massive so that price can't get bigger dump. Competitors might share total crypto marketcap
That remains to be seen
We don't know how much money has actually flown into other alts and especially into Ethereum. If Ethereum founders are really holding 60% of premined ETH tokens (I don't know if it's true or not), that means its "market cap" is even more deceiving that it is on average between altcoins. If you premine most of coins while only some get actually traded with the rest being locked (which seems to be the case with Ethereum), you may claim insane market cap but it won't have any meaning and bearing on reality (apart from luring in inexperienced folks)