Is Ethereum and other altcoins taking over and replacing Bitcoin?
If so, how fast will this happen?
I don't think it can happen. Because bitcoin community itself is bigger and bitcoin already to be core of cryptocurrency. Not that easy to altcoins to replace bitcoin although it have more price than bitcoin. In past there are some coins which have higher price than bitcoin but never replace bitcoin.
Altcoins will never be too relevant like what bitcoin do. So it will never take over bitcoin's position today nor in the years to come. Altcoins prices may be surging these days but believe me its temporary. Once the market turns into bearish, prices will drop off and even bitcoin do that. But knowing the fact that bitcoin dominates the crypto world, it will never be that easy to replace it.