I'm unclear on what you think they could do. Is your idea here that Google is so smart that they can break end to end encryption? If so, we've got bigger problems.
It isn't fair to presume that everyone shares your lack of knowlege on a subject, and it's simply incorrect to presume that because you don't understand something that it cannot be safe or reliable.
What they say today about end-to-end encryption seems like it should work fine from a technical point of view. It is entirely possible the Google is very good about this, and when implemented, it might work perfectly as stated today.
But I'm not talking about incorrect or correct and I don't care about fairness in presuming whoever's intelligence either, because the thing I'm talking about is more important, which is risk.
Large companies taking on big tasks that you don't pay them for is undeniably risky for many reasons. One, they screw it up today. Two, they don't screw it up today but they change it tomorrow. We know this because many of these companies have done things like this before.
It isn't fair to presume that everyone shares your lack of knowlege on a subject, and it's simply incorrect to presume that because you don't understand something that it cannot be safe or reliable.