When introducing the story of Miriam, Rashi notes that it is juxtaposed with the story of the spies speaking ill of the land because the spies saw what had happened to Miriam, yet failed to learn a lesson about evil speech.
The association is bizarre, and very problematic as a source for the lesson of not speaking negatively. Miriam spoke out against a human being – and the greatest to walk this earth to boot. Why would they apply the lesson to insentient, inanimate land?
The Rambam teaches that the greater a person is, the greater exercise of humility required. The character appraisal the Torah gives of Moshe is emphatic:
והאיש משה עניו מאד מכל אדם אשר על פני האדמה – Moshe was more humble than any person on the face of the earth.
This may seem a little bit hyperbolic – but actually, indicates his level of humility – he made himself impervious to personal sensitivity – like the ground.
The lesson they could have taken on is suddenly not bizarre at all. They ought to have taken heed that Miriam spoke ill of someone who was totally detached, and genuinely did not care – not only did he completely forgive her, he immediately prayed for her recovery. This being the case, we are able to grasp the juxtaposition of the two events.
There is a phenomenally difficult, but very important lesson about sensitivity in speech here. In both cases, the error in speech was much more subtle than a straightforward, nasty piece of gossip. Yet Tisha B’Av and all tragedies in Jewish history have since ensued as a consequence.
That the level required here is beyond us may be a valid observation, but think of the reverse; what with how powerful our speech clearly is, what could be achieved with dedication and perseverance?