or a business owner wants to pay his workers monthly salary in bitcoin and he doesn't want to log in every time he needs to do that? He just simply set up a contract that will send monthly salary to some predefined address every month.
If you actually refer to "smart contract", Bitcoin script doesn't have OPCODES which let people automatically send Bitcoin. And as other user mentioned, frequent change is needed due to various reason.
Another possible use case is to prevent any unwanted transactions from coming through. Like only whitelisted addresses can send funds to a specific address, transactions from any unknown address will not be mined in this way, and the receiver address can pay for the transaction fees.
It's technically impossible. What the wallet could do is showing notice that the transaction doesn't come from whitelisted address.
Also, using and securing manmonic phrases or privet keys seems inconvenient for many of us, so using NFT's or some other possible ways to make your bitcoin wallets back up grandma-friendly will be a major development for wide adaptation of the bitcoin network.
Average people don't have to interact with private key directly. Mnemonic isn't that inconvenient either since usually you only need to interact with it when you create or restore your wallet. Securing isn't that hard if they simply write it down on paper (or other medium) and store it on their house.