You would lose some type safety and possibly cause confusion.
For example:
val iThinkThisIsAList = 2
for (i <- iThinkThisIsAList) yield { i + 1 }
I (for whatever reason) thought I had a list, and it didn’t get caught by the compiler when I iterated over it because it was auto-converted to an Option[Int].
I should add that I think this is a great implicit to have explicitly imported, just probably not a global default.