Ethereum's long-term security model is Proof of Stake.
At best, it's economic rent with holders getting free money from workers. At worst, it's a game of musical chairs that only people who don't use the money can win. Profits from securing the network ought to automatically adjust towards zero.
PoS is why Ethereum bailed out everyone with money in the DAO. PoW is why Bitcoin didn't bail out anyone with money in MtGox. The cost of auditing smart contracts ought to be internalized to the people using them, not an external cost for all users.
Upgrading Bitcoin is a great idea. Ethereum's flexibility has been proven useful by the huge variety of projects that run on it. So we should support BIP 300 and get some of that. But I would not consider Ethereum's security model to be an upgrade.