Bitcoin script provides a no elegant way to invalidate old states. There are various solutions to this in lightning network tx but they end up requiring actively watching the network in case an old state is broadcast. If you have a better mechanism for invalidate the previous state, the whole system can be massively simplified.

You mean, every transaction causes an on-chain ethereum change? That seems to miss the point?

Mainly, having account balances and arbitrary data storage, instead of bitcoin's unspent transaction outputs. See this article by Ethereum's inventor: https://medium.com/@ConsenSys/thoughts-on-utxo-by-vitalik-bu...

Also, having a convenient scripting language instead of dealing directly with opcodes, like you have to do on Bitcoin.

Since we're only talking about a single utxo in either case, the difference seems moot.

And writing and testing the opcodes took less than a day; that's not the hard part! (Though getting the convenient opcodes into bitcoin was definitely non-trivial!)