That relies on the assumption that they will not suffer a tremendous loss due to a rogue employee, a software bug or a malicious hack.
If they found themselves in a situation in which they are certain to go out of business as soon as people find out that they are in trouble, that motivation would cease to exist. It would come down to ethics, reputation, pressure from investors, etc.
Sure, Bad Things™ could happen at Coinbase like they did at Gox. However, I think it is reasonable to trust Coinbase more than Mt. Gox. If you looking to Mt. Gox's history there are many signals that suggest general mismanagement and dysfunction.
This is in no way rigorous but consider User Interface design as a rule of thumb. Gox had an ugly unintuitive site. Coinbase on the other hand has a polished UI. This attention to detail suggests that the people who operate Coinbase are very different from those who operated Gox.
If they found themselves in a situation in which they are certain to go out of business as soon as people find out that they are in trouble, that motivation would cease to exist. It would come down to ethics, reputation, pressure from investors, etc.