If I choose "No default configuration" when resetting a MikroTik router, will I lose that default configuration "forever"?

No they are baked into the firmware.

You can see all default config scripts in the docs here: https://help.mikrotik.com/docs/spaces/ROS/pages/167706788/Default+configurations There are different scripts for different devices

Or on a terminal you can even print the exact script which runs if you don't enable "no default config" on reset:

 shell

    
/system/default-configuration/print

  

Be careful, if you reset without default config, the DHCP client and server won't start automatically, so your device won't have an IP address, so you have to connect with Winbox via MAC address