I can't speak for everybody, but there are several reasons to purchase assets (no particular order):

Quality - paid assets are typically better quality than free

Quantity - you can usually get more variety in individual pieces on paid assets

Consistency - it can be difficult to find "matching" free assets

Specificity - it can be difficult to find specific/unique pieces among free assets

Exclusivity - if you want assets that nobody else can use....

Convenience - scrolling through thousands of free assets takes a LOT of time

Patronage - some people buy assets to support artists and/or encourage them to release more (sometimes free) work

disclosure: I've purchased on GameMaker market, Unity Asset Store, GameDev Market, and Graphics River

Here you go:

https://www.assetstore.unity3d.com/en/#!/list/39530-complete-mobile-proj...

Goal = the amount of "currency" you need to unlock the next work

Base = total "currency" in each given world

N = World/Goal number

World = each game map

Adjustment = penalties and/or bonuses (can be linear, geometric, random, or unlocked via game play)

For each World(N)

Goal(N) = Base(N) + Goal(N-1) + Adjustment

Set your menu/login screen to World(0) and hardcode Goal(0) = 0.   That means you automatically "unlock" the first world.  Subsequent worlds are "unlocked" as follows

Goal(1) = Base(1) + Goal(0) + Adjustment

Goal(2) = Base(2) + Gaol(1) + Adjustment

 

And so on

Hope that helps!

Love the concept, but would agree with @Spring about the "post-apocalyptic" description, as that makes me think of "Road Warrior" or something right after the fall of human civilization.

Since you're essentially starting anew, maybe  a "savage land" or "pangaea ultima" theme? https://science.nasa.gov/science-news/science-at-nasa/2000/ast06oct_1