If we compare them now , it is obvious that bitcoins are the best crypto currency and alt coins don't get a stand in front of it. People use bitcoins to buy altcoins , alt coins was started because of the success of bitcoins , it became the first crypto currency to gain value of $4000. Bitcoins have a big community behind them and the users are increasing in numbers ,
But we cannot say about the future , alt coins like ethereum have good potential, it may seem impossible but can happen in future .
Yeah, current bitcoin is still number one but the future is unpredictable because the competitor with bitcoin is very much, if Bitcoin does not try to improve and upgrade in the future, the competitors will be defeated and replaced Bitcoin. This is a very fierce market, any time Bitcoin stop development, altcoin will also take advantage of the opportunity against it.
You got the point bitcoin still not improve and every year it will be more worst since the transaction fee is really high these days i think that bitcoin fees can be increase more in the future that pushing other bitcoin holders to use other altcoin just to save in paying higher fees..
For now there are still no altcoin actually can be accepted by many merchant so still bitcoin will remain that acceptable and support by many merchant..
That is why altcoin has no chance if they don't let other merchant accept ethereum as payment..