This is very obvious. Bitcoin is already leading the market with a cap of almost 50% of the entire cryptocurrency market. This dominance is not ceasing any time soon and Bitcoin will lead the market for years. Ethereum will close in the gap when it launches its Ethereum 2.O but that will not even surpass BTC dominance.
Bitcoin dominated a lot of other currency long time ago, but bitcoin has a lot of work to do for itself to develop and become better over time. It is somehow the most popular and best cryptocurrency in the market but it still experience downward in price depending on the factors. I guarantee you to work on some altcoins so that you have a back up, for me, next to bitcoin is ETH.
ETH is really effective also in the market, you can get profit from it if you manipulated it properly, but not as high as bitcoin. Small progress is still a progress, although ETH can't give you huge profit like bitcoin, that altcoin has a potential to become more popular than bitcoin, we don't know what can this coins can do in the future.
Never underestimate other altcoins, but for the mean time, prioritize bitcoin because it is really effective in the market.