Of course you can write anything in R but that doesn't mean it will be possible to maintain the codebase for the next 10 maybe 20 years including dev team changes.

In other words R is wrong tool to use for building the software just from maintainability standpoint.

I agree that in general one would not use R to engineer software. However I disagree regarding low maintainability.

I've seen very complex R packages that have been around for 30+ years. They are actively maintained. Applying good design principles, unit testing, modularity - all that is very much doable in R.

And you will want to write software in R, especially if the purpose is to support the very activity of working with R packages and to expose the R statistical environment over HTTP. Case in point, the software I link to, in my comment above.