Why was the punishment for the sin of the spies so severe?

Question

Avot Derabbi Natan 9:2, commenting on Bamidbar 14:22, enumerates the 10 times that B'nai Israel tested G-d in the desert. The Mishnah states that G-d did not punish them until they spoke lashon hara about the land of Israel.

This implies that speaking lashon hara was even greater than the sin of the Golden Calf which was idol worship.

I mention that sin specifically because we see similar phrasing in Moses plea to G-d for forgiveness, such as mentioning that Egypt will think that G-d pre-planned to destroy his people in the desert. We also see a partial repetition of the 13 midot.

So G-d forgives the sin of the Golden Calf but not the sin of Lashon hara. Why was this sin unforgiveable?

Answer 1

This article offers an insight:

The reason why the punishment here is seemingly so harsh is because of the gravity of the sin of lashon hara that lashon hara drags back previous sins and makes one get punished more stringently for those too. Thus, via the lashon hara Bnei Yisrael did not just get punished for this sin, but in essence for parts of the previous sins too; HaShem says (14;22) ‘they tested Me ten times,’ (referring to the ten sins Bnei Yisrael did in the desert) since they now get punished for the complete unit of ten past sins due to this lashon hara. This might also be the explanation of a complicated Rashi (14;33) which says that when Bnei Yisrael sinned at the golden calf, HaShem decided the decree of wandering around for 40 years in the desert, but waited until the sin of the spies to utilise it. The point is, according to the above, that the lashon hara dragged back the deserved punishments for past sins.

Answer 2

Actually, the punishment was made lenient by being spread over the next forty years so that the people (between 20 and 60) would not die at once. As stated by many people including the Kli Yakar and Rav Hirsch as I explain in Why were the Jews punished for 40 years if the spies were the ones who were there for 40 days?

The punishment was that all those who were in the army (and were thus the ones who were to enter and fight) would die at the end of their service (at age 60). Everyone above the age of 60 would die at their normal time of death and the people below 20 would live to enter the land of Israel.

By delaying the death of the army until age 60, Hashem allowed the youth to mature and be ready to enter the land.

Similarly, Hashem postponed the punishment for the Golden Calf to have some element of that punishment occur whenever Bnai Yisrael sinned and were punished in the future.