it can't!
because bitcoin will always remain a decentralized currency.
and ethereum will always be a bad platform for smart contract which was never supposed to be a currency. and not to mention the alarmingly high transaction fees of its network (getting closer to $1) which comes from its terrible scaling issue.
Ethereum block sizes scale up linearly over time so eventually it will fix itself, and PoS offers more scaleability than the current PoW they use. I think eventually it will reach a higher market cap than btc but i don't think btc will collapse in the same way all other shitty alts haven't collapsed.