I thought the game was delightful, short as it is. While all of the additions you ache for would definitely improve on the game, I had a wonderful time exploring the island and felt the ending wrapped things up just fine. (Unlike, say, Firewatch. Ech.)

I loved the ending. I did no quests except stickball and had the minimum amount of feathers. When I glided around the mountain at the end it was amazing. I don't think the phone call is the end. The game's climatic finish is when you soar down the mountain and see all the challenging parts, knowing that you conquered them.

A Short Hike is incredible because the player's intrinsic  motivations line up so perfectly with the Claire's 

it's naturally satisfying to do the activities which contribute to your ability to conquer the mountain.

The length of the game is literally in the title A Short Hike

like the Tibetan Buddhists painstakingly assembling gorgeous colorful Mandalas one sand particle at a time,
only to then destroy the Mandalas at the end of the ritual

the game is a  lesson in Impermanence

all of existence, without exception, is "inconstant"

All temporal things, material or mental, are in a continuous change of condition, subject to decline and destruction.

All physical and mental events are not metaphysically real, that they are not constant or permanent, they come into being and dissolve.