*Since you specifically mention TWRP in your question, I'll just use it as my example. Note that [CWM][1] or any other possible Android Recovery project will also apply.*

It looks like there's a slight misconception at play. I'll just drop a few clarifying notes here:

- TWRP is indeed an image to be flashed.
- Flashing TWRP does not overwrite your bootloader.
- TWRP is not a bootloader replacement - it is Recovery software. It is a drop-in replacement for Android's stock open-source Recovery Mode, and therefore has the access needed to do things like

  - Create/Restore backups
  - Root devices
  - Install official updates

So the short answer is: **Yes! It is indeed possible to root without overwriting the stock bootloader!**

A few more notes:

- TWRP (And [CWM Recovery][1]) is a mature (Several years old and quite stable) Open-source project that is used by a huge portion of the Android community and considered very trustworthy.
- Flashing TWRP will replace the contents of `/recovery/`.
- You generally need to use the stock bootloader in order to select the boot-to-recovery option in order to use TWRP.

  [1]: http://www.clockworkmod.com/rommanager