Private IP support for Cloud SQL is on the roadmap.
The reality of development is that some things are easier to do on a new product. It's not a matter of communication.
It's a double edged sword. When you don't limit connections and customers push their servers beyond their limits, some of them then ask why Google didn't do anything to prevent the situation from happening. Ideally, this would be solved by having better failure modes but that's not really the case today.
Most product decisions are not set in stone and can be changed if there's sufficient evidence to suggest that they do more harm than good. There's a lot of work that goes on behind the scenes to keep the service running smoothly and unfortunately/fortunately customers are not exposed to them. Some things that may appear to be simple on the surface are not when you take things like supportability into consideration.