There weren't any risk before apart form the kotlin plugin not working with beta versions of the gradle build plugin. Using kotlin in Android is like using a library, and you don't have official support from Google for all popular libraries used in Android.
The risk was that you might end up with a code base that you cannot find developers that will work on it/maintain it in the future. The Android support should pretty much guarantee that there will be enough developers familiar with it in the future.
If you are an Android developer you should be aware of kotlin long time and and also tried and use it. Good Android developer knew kotlin before, now you will face more noise to find them.
It is like hiring something who doesn't know the most used libraries in Android. And the risk is exactly the same. The language is not difficult to learn, not much more than a mid-complex library.
But Kotlin was farrrrrrrrrr from a "most used library". The risk of an unsupported language being used is much, much greater than using an unsupported library. Libraries are non-trivial to remove from your code, but should not be tied to your code to the point that you will be required to completely rewrite your application like you would if you ran into issues with the language it's written in.
Say so to the people who is using RxJava or similar, whether they need to rewrite everything or not. In every program you need to take architectural decisions that affect how everything is build and a change on them could mean to rewrite everything. It can be a library or the programming language.
JetBrains was supporting kotlin for Android, the same people that build IntelliJ idea (the base of Android Studio). But who is supporting RxJava? Imagine that the react community wants to make a move that will harm Android devs, so they change all the Rx*, including RxJava.
Using kotlin wasn't a risk. Google said it will make it first class citizen, but do you really trust Google in the long term? They close products very quickly. I feel safer knowing that JetBrains is behind kotlin and not Google.
There are risks other than just not being able to build it.
If I have an app with a codebase in a language that very few people know, that's a risk. What if my main developer leaves and I need to find someone else to continue working on the code base?
You can now bet that a lot of people will be picking up Kotlin as a result of the latest announcement.
