As you are a fervent advocate of the safestring library you should read the study provided by the author of the saftestring library ...
For small sketches with a few Strings, just use them as convenient
For small sketches with a few Strings, just use them. The good news is if you are using an UNO or Mega2560 or another AVR based board, then using Strings is extremely safe and won't crash you board*, even if you run out-of-memory.
If you run out-of-memory, you will just not get all the text in the Strings that you expect. The program will continue to run. See What happens when UNO runs out of heap memory below.
Other boards will have more memory available and so you are unlikely to have memory problems using a few Strings.
So your point is a moot one, even under the opinion of the developer of the SafeString library himself and if you follow his "Guidelines for Using Arduino Strings" you'll stay out of trouble.
You don't need the SafeString library just for strings.
The author shared his own work which can make life easier (at the expense of a steep learning curve of something custom) as he said
If you want to keep complete control over your memory usage OR you are working with a library that returns a char* OR you want a richer set of readers, parsers and text functions OR you want detailed debugging/error messages, then you can install the SafeString library from the Arduino library manager
The value add is more there if you are willing to adopt his APIs for further features than replacing the String class.