Yep, that's the main goal there. Getting changes through there hasn't been easy as there are many different things that share the release cycle and dependencies. urdf hasn't been moved yet, you could push the changes in now.
Changing from `boost` to `std` `shared_ptr` in e.g. ROS-M would require all consumers of that API to update their code. Each package needs to make that decision on its own. But for many "core" ROS packages that might simply not be feasible or desired. So this downside needs to be taken into account when considering such a change.
That was a thorough comment @tfoote wrote and a good reminder of the rationale. I don't know if ROS-M will still target Ubuntu 16.04, but if not, gcc6 will be on all Ubuntu distros already ([starting with 16.10](https://packages.ubuntu.com/yakkety/gcc)) so the possibility is there.
Following the previous targeted platforms, ROS-Melodic will target ubuntu 17.10 (last LTS - 1) and ubuntu 18.04 (last LTS) so they will both have gcc >=6.3.0.
The compiler support is not going to be a blocker here. I think a discussion with the community on the next iteration of REP3 for ROS Melodic is required to find a good compromise between leveraging new features v.s. stability of the APIs for all downstream packages (as pointed out by @tfoote and @dirk-thomas).