The witnesses are here to help us to believe.
Let’s first translate what Deuteronomy 32:1 really says. The Torah speaks of the sky and the ground level, not of Heaven and Earth. They are mentioned prominently already in Genesis 1, and I see no reason to modify that translation further down in Scripture.
The following verse, 32:2, talks of rain, dew, showers, and drops—falling from the sky to the ground. The basic meaning of these verses must be that Moses wants his speech to come down as life-giving as water. Still in the desert, poetically, he calls upon the sky to open up and the ground (people) to receive his words as if it’s quenching rain. On top of this, in 32:4, Moses speaks of G^d as rock-solid; rocks gave water too (Exodus 17:6, Numbers 20:11, Deuteronomy 8:15, Psalms 78:15, 105:41, and 114:8). Only in verse 32:5, Moses begins to admonish us.
Also, do the sky and earth have an ear? We must say that, just like the ground swallowed up Korach as if it had a mouth (Numbers 16:30, 32, 26:10, and Sayings of the Elders 5:6), the sky should hear Moses’ words as if it had an ear from which the rain can come down, and the ground should absorb his words as if it had an ear through which to absorb rain and dew.
The words of our Chief Commentator Rashi on Deuteronomy 32:1 are ingrained in our minds. He explains that the dying Moses calls upon the sky and the ground to act as witnesses because they’ll stay around after him. That is important, especially in the Ten Days of Repentance, when we read this. However, the basic meaning must be what I shared above.
Just 3 verses earlier, but in the previous Torah Portion, in verse 31:28, the Torah does name the sky and ground as witnesses. Yet, there we see they should testify for the Elders of your Tribes and your officials. Not against future generations! And, again, this must mean, basically, that the sky and ground testify to them, the sky in its immutability of the L^rd’s eternity, and the order at ground level (see the Creation Story) of the Creat^r.
Very prominently, just before, in verse 30:19, Moses does call up the sky and the ground as witnesses. But not really. It says: My witnesses are for you the sky and the ground: I gave you this life and this death, this blessing and this curse, and you are advised to choose life, so that you and your offspring may live. Again, these witnesses are to explain how we could believe in G^d. They are not to witness against anyone. The verse does say ‘today,’ which I believe to mean: every day from now on. It’s a lesson for eternity. But what we see in the sky and on earth should help us believe.
We recount twice daily that when you obey G^d, “then your days and your children’s days will multiply to be all over this Land [same word as ground] that G^d swore to your ancestors to give to them as long as the ground will face the sky” (Deuteronomy 11:21). That’s a long time.