Why bitcoin? For starters, let us remember that bitcoin was the first cryptocurrency ever. And the technology behind it has been the basis for all other cryptocurrencies around. So i guess it is only fitting to have bitcoin lead the crypto rally in respect to its great contribution to the crypto world.

Secondly, we all know that bitcoin is the most popular, the best valued and the most adopted cryptocurrency till date.  Though we have altcoins in competition, bitcoin remains the major brand of them all, and that, without a doubt.

Thus, it would definitely be hard to picture the world of crypto without bitcoin on it, but still, one can not discount the possibility of bitcoin exiting the scene.  As with any other technology related creations, a life span is always an expected integral part of the design and at times, inevitable.

And to address your question whether is it possible for an altcoin to be number 1 instead of bitcoin, i'd say yes.  We all know that top altcoins are continuously improving their utilization and design to be better than bitcoin and infact has made themselves faster in transacting and with more robust scalability. Thus, it will be no surprise in case an altcoin takes the lead in the future.