Well then nothing is secure.

At least as far as a whole system goes including the soft squishy components designing, developing and operating it.

Even for cryptography the only absolute proven secure method is the one time pad which has very real practical flaws and risks if you try to operate it in the real world.

The threat model here is people leaking their own Coinbase passwords (via phishing, compromised PC, etc.), so the 48-hour delay and out-of-band verification ought to actually help.

So, 72-hour delay is even more secure?

Yep, and if you dump all your coins into an account with no private key, nobody can get your bitcoins ever, so that's really what everybody should do.

All sarcasm aside, Bitcoin is becoming more and more centralized everyday.

Of course that's not true. Convenience / security trade-offs are common, and lie on a spectrum.

Hardly a valid point if it's money related, but that's just me then, as the downvotes imply.

Even if it's money related. Carrying physical cash around with you is convenient, but not as secure as storing it in a bank.