Coinbase could be that big in 2020, but remember that cryptocurrencies dont need banks to hold our money. Even though more people are jumping into this cryptocurrency market, that doesnt mean they are holding their coins on an exchange. Lots of people may be keeping most of their coins in offline wallets.
Its true that we need exchanges/banks like Coinbase to buy and cash out our Bitcoins, but thats all I use them for. As soon as I buy, I move the coins off Coinbase and into a paper wallet where it will sit for years.
Most people in the forum are interested in trading and take active part on that which means that an important part of their bitcoin holdings needs to be in an exchange, this is worrying since exchanges can be seized or hacked and then you lose your coins with no hope of getting them back, and when we take into account that most traders are going to be losers in the long run then putting your money in an exchange makes even less sense.