A meta-design point why we want to allow implicit conversions in arguments but not elsewhere: Implicit conversions are sometimes useful to support ad-hoc polymorphism. We already have two alternatives in our toolbox to address this: overloading and typeclasses. But overloading can lead to a combinatorial explosion of method definitions, typeclasses are more heavyweight, and neither can deal with variadic arguments. So allowing implicit conversions as a third way to support ad-hoc polymorphism seems reasonable.
My thesis is that that’s it. There is no other widespread, legitimate use case of user-defined implicit conversions in simple Scala 3 code. For more advanced code a language import is a reasonable requirement. Now, prove me wrong. 