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Bitcoin dominance is not decreasing and it's an obvious statistics that you can get by looking at both market cap and how much liquidity bitcoin has compared to the entire crypto ecosystem. Basically when bitcoin pumps it has a huge influence on the general market and that is part of the factors that determine the dominance of a particular currency over others.
Dominance can simply be decided by dividing the total market cap of crypto by bitcoin's and right now it is obvious that bitcoin remains the one on top. The past year, I do not think we really saw altcoins rise so maybe people now think that bitcoin has and will always dominate the market. And maybe that is right but dominance can definitely shift towards altcoins more and in fact has happened before. It is just that currently, the dominance remains to be on bitcoin's favor.