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but it is all about the context that the term "dominance" is being used in. people aren't just using the term to compare market capitalizations, they are using the term "dominance" to say bitcoin is losing its ground and being replaced by an altcoin.
People have strong tendency to simplify complex issues to make it easier to digest, that's not necessarily a bad thing. But when Bitcoin was actually under real threat of Ethereum taking over the 1st place, the most quoted source was [ur=https://www.flippening.watch/l]flippening watch[/url], which compared multiple other metrics other than market cap. People seem to understand that there's more to it than just market cap.
What's funny, if you dismiss market cap entirely, you'd have to admit that, by other
measurable metrics, Bitcoin briefly lost its #1 badge at one point (got overtaken by ETH in trade volume, number of transactions, node count + few more).
besides when you compare market capitalizations you should also consider that fact that market cap of a lot of altcoins is FAKE because their coins are not in circulation. for instance the coins with premine that have not yet released the coins by dumping them on exchanges. if they do it the price of them will fall and consequently their market cap will fall making it more real.
You could say the same about Bitcoin. Aren't Satoshi's coins (~1 million) also included in market cap? You have to use some number and the actual number of coins in circulation is impossible to tell.
But that's why I say it's best to watch dominance by comparing Bitcoin to #2 coin, rather than looking at total market cap share.