Why did Allah praise those whose scales are heavy with success, even though the stages of the Day of Resurrection have not yet ended, and one of them might enter Hellfire due to injustices against people, such as backbiting?

The Day of Judgment is the Day of the Great Terror, and it has stages and stances. Whoever overcomes a stage or a stance deserves congratulations due to the horror and severity of the situation. He may have been given glad tidings of Paradise before that, but this does not mean his immediate entry into Paradise, except for those about whom a text has been revealed stating their entry without accountability. The terror necessitates glad tidings for those who overcome a stage; Allah Almighty says: "Then those whose scales are heavy [with good deeds] - it is they who are the successful" and He says: "Then as for he who is given his record in his right hand, he will have an easy reckoning."