If CEX can double spend, or 51% attack or whathave you......the problem seems to be with bitcoin itself, not any one user. CEX seem to be good guys, what happens if bad guys develop the same power.
Good or Bad, they are subject to coercion.
Centralization of power is bad for bitcoin, it destroys its value, it adds trust relationships where they don't belong.
CEX is unwise for allowing themselves to get to this point, that alone, even if it doesn't make them "bad" and "untrustworthy", they need to be better than this.
They do not have the power to resist the coercive forces at play in the world, and either have too much hubris to see the truth of that, or are already compromised. Either way, there are evidently necessary improvements needed. Yes, in the protocol, as well as in the wisdom and responsibility of those that would hold its reins.
