On quantity change, create individual line items of the same item in Drupal Commerce cart
It would be possible to create a new views handler based on commerce_line_item_handler_field_edit_quantity that worked the way you want, but IMHO it would be weird from a UX POV to enter 5 into the quantity textfield of 1 line item, submit, and then see 5 distinct line items with quantity 1.

A simple alternative would be to modify the cart view to have something like an Add 1 more button:


Remove the quantity field;
Add a relationship to the product;
Add the product's add to cart form as a field;
Configure the field to not combine products (and you can also modify the text here, eg Add 1 more).


I haven't tested this, but I think it should work.