Rav Elyashiv actually held that a wig cannot be longer than the highest vertebra on the back. And he held that the tichel is the ideal head covering.
On page 133 of the Unique Princess:
The Laws of Modesty
A woman’s Behavior in the Presence of men
Written by HaGaon HaRav Yitzchak Zilberstein shlita and approved by Maran HaGaon Rabbi Yosef Sholom Elyashiv, shlita
– If she wears a sheitel, wig- the sheitel must look like a wig. It must be styled modestly and must not be longer than the highest vertebra on the back
– IN THE VIEW OF MARAN HAGAON RABBI YOSEF SHOLOM ELYASHIV, SHLITA, IT IS PREFERABLE TO COVER ONE’S HAIR WITH A KERCHIEF RATHER THAN A WIG
That’s why his Rebbetzin wore a tichel – because Rav Elyashiv held that this is the best way to cover the hair