Hi,
Change the line:
echo $this->element->main->product_name;
to:
echo $this->element->product_name;
in the view file product / show_default via the menu Display>Views and that's how it will display.
We actually had it like that when we first released HikaShop. However, after several feedback saying that even if the default variant is preselected, it would make more sense to not display the variant name when reaching the product page, we changed it to the way it works now.
Also, by default, with several characteristics and the dynamic charactersitic selection setting activated, no variant will be preselected by default on the product page. So in that case, it make more sense to display the main product name by default anyways.