What was money's killer app? Bitcoin is money from the ground up. It's going fantastically fast. As for smart contracts, these are the basis of Bitcoin. A permissionless ledger inherently requires smart contracts. Their killer app is a a distributed ledger. There are already a few different contracts that can be made on Bitcoin that allow for different custodial schemes.
What people are selling as far as DAOs, NFTs, and "smart contracts" where they mean are better and more sophisticated than Bitcoin, can all be favorably described as the application of Bitcoin's critique of money to the issuance and governance of other tokens representing other forms of property, and are inherently more complex. Least favorably described, they are scams to boost leverage powered attacks. The middle of the road might be that they're the naive notion that building more complex constructs on top of what Bitcoin defined is immediately achievable without compromising invariants that matter for useful money.
All to say that the "killer app" critique of Bitcoin probably ins't the best lens, and that if it's more complicated than custody and payments, especially for people where that isn't readily available, it's probably further out than is survivable for a single company requiring the kind of trajectory of those that create "apps".