Coinbase is just an exchange; plenty of them have come and gone since bitcoin has been around. Mt. Gox was much bigger on a percentage basis than Coinbase is now when it went out of business. There are plenty of other exchanges and other ways for people to buy and sell bitcoin [1][2].

Bitcoin is a global network of tens of thousands of full nodes verifying the block chain and miners; there's no feasible attack for this.

It's been 12 years; I'm sure it's been attacked every which way possible in that time. The only way you can stop bitcoin is to shutdown the internet and that's not going to happen.

One mantra in the Bitcoin community to live by: "Not your keys, not your bitcoin."

Anyone with a substantial amount of bitcoin should transfer them from an exchange (or Cash App or PayPal) to a wallet they control, which isn't that difficult. There are plenty of hardware wallets to enable this and n of m multisig solutions, so there isn't a single point of failure.

[1]: https://keys.casa