Short version:

Instead of trying to build a smart contract with the very limited scripting language of #Bitcoin, which doesn't give you much functionality for very important security reasons, instead you treat simple yes/no script as its own computation - ie. use it as 0s and 1s for an abstracted program on top of it - that way you can literally write an entire program into a huge hash tree of random yes/no script executions tied to hashes from an external program. That way all of the participants can enforce the external program, and its completely agnostic to the code. You can literally write *anything* into it and it will enforce the ownership of actual btc.

Possible now, permissionless, and the programs are entirely off chain, not only does bitcoin need to do no computation, but its also private.