While I like to use the latest and greatest myself whenever i can, I think you have it a bit backwards. If the baseline for ROS2 development is C++11 or 14 or whatever, that should not prevent you from using it with more recent standards in your own code. The inverse however, is not true, so ROS 2 should continue to support the initialially chosen baseline dependencies for some period if time. I find that more often than not ROS is not the only dependency.
Long term support with stable dependencies is important. I believe the distribution model of ROS 1 works quite well in that case.
In any case I also agree that C++14 at this point makes sense for ROS2 beta.