Im disappointed too. Well, I have a couple of realistic suggestions below as food for thoughts ... (some amateur experience with Kendo in the childhood)

* Add jagged ribs that prevent the attackers from grabbing your unbrella by bare hands. When grabbed, pulling it will cut their palm

* Make the fabric shark skin that similarly damages their palm when rubbed

* Make it actually heavy not light

* Make the grip thicker

* Make the grip longer enough for both hands

* Add a grip guard. This actually cause a stronger piercing force because it pushes against the grip guard rather than relying on the palm friction

This [1] may be indeed practical.

[1] https://www.amazon.co.jp/%E3%83%A6%E3%82%BF%E3%82%AB%E3%82%A...