The rebuke, no matter how harsh, is always given to people who have ability to correct the problem. The harshness may be indicative of the urgency to do so.
Evidently, the words imply that Moshe Rabbeinu foresaw and knew that, unless people change their attitude, there would be future exiles of the Jewish people. There are several places in the Torah and the commentaries where this idea is repeatedly stated.
Source 1. When Moshe spoke with Hashem at the burning bush he says Shemos 4:13:
But he said, “Please, O my lord, make someone else Your agent.” I.e. make someone else Your agent Lit. “send through whomever You will send.”
Targum Yonasan ben Uziel translates this verse:
And he said, I pray for mercy before the Lord. Send now Thy sending by the hand of Phinehas, by whom it is to be sent at the end of the days.
In other words, exodus from Egypt was supposed to be only the first one. This is also implied in the Rashi's comment:
Another explanation is: by the hand of some other person whom Thou wilt be pleased to send — for in the end “I” shall not bring them into the land of Palestine nor shall “I” become their deliverer in the future — and You have many messengers! (Pirkei DeRabbi Eliezer 40)
Source 2 When Jewish people sinned with making the Golden Calf some of the commentaries explain the image of the calf from Maase Merkava in Yechezkel 1:10
Each of them had a human face [at the front]; each of the four had the face of a lion on the right; each of the four had the face of an ox on the left; and each of the four had the face of an eagle [at the back].
The human form is known to us from the image of keruvim i.e. the mishkan, where the image of human-form keruvim was present inside Kodesh haKodashim. The right side usually symbolizes geula, revelation of Shechina, as Hashem took Jewish people with his right hand out of Egypt. Instead the Jewish people chose the image of the ox from the left which symbolizes galus and distancing of Shechina.
Source 3 In the episode of the bitter waters when Moshe rabbeinu hit the rock, some of the commentaries explain that Moshe did not want to raise the new generation to the level of their parents. For just like their parents faced the test with the Golden Calf and failed it, Moshe rabbeinu thought that the new generation would not be able to successfully emerge from a similar test. In other words, Moshe rabbeinu and the Jewish people accepted the idea of future exiles as an alternative to attempting immediately correct the sin of the Golden Calf (long short road).
Source 4 The Torah section that we read on Tisha b'Av Devarim 4:25-40 speaks about future exiles not as a conditional, but factually.
When you have begotten children and children’s children and are long established in the land, should you act wickedly and make for yourselves a sculptured image in any likeness, causing your God Hashem displeasure and vexation. I call heaven and earth this day to witness against you that you shall soon perish from the land that you are crossing the Jordan to possess; you shall not long endure in it, but shall be utterly wiped out. Hashem will scatter you among the peoples, and only a scant few of you shall be left among the nations to which Hashem will drive you. There you will serve gods of wood and stone, made by human hands, that cannot see or hear or eat or smell.
Therefore, one should understand the words of Moshe rabbeinu not as a harsh rebuke, but the fact of the objective vision of the future. Given that vision, he is teaching Jewish people about how we can survive and carry the message of Hashem who took us out of Mitzraim.
