The proof would be higher profit margins.

Even theoretically, I don’t see a high barrier to entry. Setting up a website and logistics operation seems doable such that there already exist many clothing retailers with similarly low profit margins.

And I don’t see what network effects there would be, any customer can trivially change the website they are buying from and similarly, any textile worker/producing company in the world can sell to another clothing retailer.

If Shein had some super advanced robots that could pump out clothes at competitive quality but at vastly lower prices, then that would be something with high barrier to entry, but then their profit margins would be higher because their COGS would be lower than their competitors.