You do not need much ETH to invest in DeFi Craze, enough to send the coins and pay Gas.
If 10 million people invest in DeFi Craze, the demand will increase by $ 200 million if each person invests $ 2 in fees.
ETH market capacity is large, $ 2000 million will not move the price, let alone 20.
DeFi Craze same as ICO Craze will end will cry
Yes, so far ETH has only played a role of paying deposits in ICOs or IEOs on the ETH platform. People only need a small amount of ETH to be able to participate in DeFi, so DeFi's success will not greatly affect ETH.