Default config is defined in the firmware. It can’t be deleted or changed (well, easily. I think there is a reseller option to have a custom default config).
The “no default config” means the default config will not be applied after the reset.
If you reset it again without checking “no default config”, then the default config will be applied.

“No default config” is very useful for applying your own config script. It gives you a blank canvas, making scripting a lot easier!

I have my “config.rsc” file that has the required configuration. And I have a “reset.auto.rsc” file that only has the command to reset the mikrotik with no defaults and to run the “config.rsc” script after reset.
“filename.auto.rsc” will be executed as soon as it gets FTPd (it’s a feature of mikrotik).
I use a bash script that FTPs the config.rsc file to the mikrotik, then the reset.auto.rsc file.
Makes it trivial to tweak the config then apply it, and I get all the config for the devices in easy to edit/diff script files.

It’ll always ask for the default configuration after hard reset. So, no, you won’t lose it forever.