Is it positive for Bitcoin if Ethereum proves that POS is way way superior? And if people actually start using Ether as a currency in shops, start using it with many dapps etc.? What if something much better than that comes up? Are you absolutely sure Bitcoin survives in the long-term?
I do believe one day Ethereum will overtake Bitcoin, but I think we may be a few years away from that (possibly next cycle). Reason being is that everyone's first exposure to crypto is Bitcoin. It's hard enough for them to understand that. After that they have a better foundation to start understanding Ethereum. Once you understand Ethereum and see the network effects it has and that all innovation is happening there it's pretty easy to see how it has more growth potential. The ability to stake is what I think will really steal from Bitcoin's "store of value" narrative though as it offers people interest and will also lock up circulation which will help with price stability.
