Is Ethereum and other altcoins taking over and replacing Bitcoin?
If so, how fast will this happen?
As everything in crypto happens in cycles, so its the same case with this ETH and other altcoins price increase. Back in early 2018 BTC dominance fell to 38%, and eventually it went up above 70% again. Now we are seeing the same thing, BTC price is going sideways for some time, that gives alts more room to breathe, therefore decreasing BTC dominance which is currently approximately 48%.
So, even if BTC dominance falls below 40%, it doesn't mean that BTC is not relevant anymore, it is just a cycle and sooner or later people will take the profits from their alcoins and put it back into BTC, which is much safer in the long run.
As for Ethereum, they aren't even in the same category. Both are trying to do different things.
Exactly this, ETH and BTC can perfectly coexist as you said they are trying to do different things.