First of all, there is no function in DietPi who could delete user dietpi, because this is a user required for some functionality and therefor part of DietPi core. Loosing the root password is as well not an issue of DietPi. And I need to disagree with your statement about instability of DietPi related to loosing passwords.
I guess you are running a Raspberry Pi? If yes, you could have a look to following post. https://dietpi.com/forum/t/reset-password-for-login/6581/5