It's not the single word alone in a line. It's a final line that significantly shorter than all the others. You want to strive for text that's mostly ██-shaped,

It's not the single word alone in a line. It's a final line that significantly shorter than all the others. You want to strive for text that's mostly ██-shaped, not ▐▀-shaped. Replacing the last space by a non-breaking one just shifts the problem around a bit (and you still have occurrences like “I am” at the end of a sentence – it's not like the final word is always something nice and long like “sightseeing”).