Doesn't even have to be a visionary executive. This is one of the decisions where a senior engineer writing the design doc one way or the other can easily make the difference in how the final product looks like.
Also, besides marketing, reduced compliance costs. Once police etc. realize that you can't provide useful data, they stop asking.
When the Snowden leaks came out it was a surprise to Google that the government had tapped the data center to data center connections, as these were private and not thought to be tapped. Eventually all traffic between data centers was encrypted to prevent eavesdropping.
Internally there was a lot of animosity towards the three letter agencies for tapping our lines, so part of the rational was sticking it to them.
Also, once you hire a security engineer for one thing they tend to be pretty vocal about other security issues. They can often stir up enough trouble that it's easier just to add the extra encryption.
Adding this encryption to Google's messenger was probably a couple person-years of effort. So, like, 0.00001% of the budget?
Google started encrypting communications across its data centers (even though they were on private lines) because of this revelation. Here’s a news report from 2013 [1] about this.
Also, besides marketing, reduced compliance costs. Once police etc. realize that you can't provide useful data, they stop asking.