Tell me about it... I agonised about whether we should take this risk. I concluded that we'd hired Mike for a reason and that I trust his judgement. AND the risk/reward tradeoff was really favourable. On the upside, we get all the advantages that Kotlin brings and on the downside we get risks that were actually very manageable (essentially: the 100% java interop allowed for a containment strategy should it not have worked out).
Happily, Mike was 100% right and it has worked excellently: the language is massively more productive, our development team (none of whom knew it before joining) learned it in no time and we're now reaping the rewards.
And, as Mike stresses in his post, just because Corda itself is written in Kotlin, it imposes no such obligation on users. You can write CorDapps just fine in Java (even if most people end up finding they prefer Kotlin )