Cryptocurrency is money that exists in Internet and it can be successful if and only if the product is digital.
Lets say that there is an ICO that wants to sell milk and they say that if you buy milk with their coin "Milco" you get it 10% cheaper.
Who prevents people from still buying with Fiat ?
No one.
People will probably still buy using fiat, the coin will be soon useless and your BTC, ETH coins will come to the owners pocket.

Haven't you heard this story the last few years, again and again?

Just think about it.