Re: valuation. The biggest contributing factor is speculation. I think it dwarfs every other possible factor. The second biggest one - fees are paid in ETH, and fees will continue to be paid in ETH. More, EIP-1559 will actively decrease amount of ETH in circulation by requiring miners to burn ETH to include transactions via base-fee mechanism.
Re: new chain / forking. Just network effect. Ethereum currently secures huge amount of value in DeFi. Any new network will not have those funds in it. There are also stablecoins. If blockchain-native assets (BTC, ETH, DAI etc) can be "doubled" by forking (e.g. BTC to BTC+BCH) it's not possible for fiat-backed assets such as USDC, USDT, EURc etc. Issuing bank has to pick a side of the fork.
Re: new chain / forking. Just network effect. Ethereum currently secures huge amount of value in DeFi. Any new network will not have those funds in it. There are also stablecoins. If blockchain-native assets (BTC, ETH, DAI etc) can be "doubled" by forking (e.g. BTC to BTC+BCH) it's not possible for fiat-backed assets such as USDC, USDT, EURc etc. Issuing bank has to pick a side of the fork.