The internet is valuable even though most of the traffic goes to a few sites. But that has nothing to do with decentralization. No one ever promoted the internet as useful because no site would ever be super popular.
The value Bitcoin provides is that it allows you to bypass government controls and other monopolies on transferring money. If you put that Bitcoin on an exchange you no longer have that ability to bypass government controls because Coinbase will lock your account if the government tells them to do so.
And the government can lock you internet access too if it wants, see China.
You are ok with 99% of people using only a couple of sites on the decentralized internet, but not ok with 99% of people keeping the crypto on a couple of exchanges on the decentralized bitcoin.
The internet has never been decentralized. ICANN and the government have always been central authorities that can control or shut it down. I'm not saying that it is good that Zuck has so much power or good that Google has a monopoly on search.
I'm saying that crypto exists to bypass authorities and people putting crypto on a centralized exchange proves that it is only about making money trading, not about doing anything useful.
The value Bitcoin provides is that it allows you to bypass government controls and other monopolies on transferring money. If you put that Bitcoin on an exchange you no longer have that ability to bypass government controls because Coinbase will lock your account if the government tells them to do so.