Is an update of Rtools in the mid-to-distant future?

Has there been any thought or consideration given to updating Rtools from
4.9.3 to a 5.x or 6.x flavor, or was the push from 4.6 to 4.9.3 difficult
enough that this should be tabled for a year or three?

Definitely no upgrade planned for R 3.4, and I expect it won't get
upgraded until it is really needed again.

The 4.9.3 upgrade was motivated by C++11 support, which was largely
missing in 4.6.3 and becoming mainstream. I don't think there are any
new features in 5.x or 6.x at this point that are critical enough to
outweigh the disruption.

That said, any future upgrades should be much easier than the last one
because we switched to a pretty standard gcc/mingw configuration. In
fact you can easily configure R to use a more recent compiler (e.g.
from msys2), which would allow you to use new compiler features
locally.