This is a somewhat-artificial case, though. In most cases I've seen in practice, even when you're using the value of the string, you're also filtering by some n

This is a somewhat-artificial case, though. In most cases I've seen in practice, even when you're using the value of the string, you're also filtering by some non-text columns, where the compactness of the TOASTed table allows you to do the filtering step on the left side of the implicit TOAST-join much faster (and keep more of the left table hot in memory); so that even despite needing to implicitly join in the TOAST records, you're only joining them in for the filtered rowset — usually a win overall.