what's preventing additions to the current set of R reserved words/symbols?

Why add an ergonomics feature if you risk breaking a runtime that hosts a huge ecosystem? Also, once you add one feature, you are on a slippery slope and are staring straight in the face of feature bloat.

And if you say that we can be smart and judicious about what features we add, how do we structure that decision process? R does not have a "benevolent dictator" having a final word in decisions like this so you are left with design by committee with all that it entails.

The big thing with R has always been the package ecosystem, in which if you want a feature you write it yourself -- as in your magrittr example. The language itself has remained close to its S roots and has successfully served as a stable platform for all the development that has been happening.