Moshe Rabbeinu's complaint
Lama harei'osa la'am hazeh ki may'az basi ledaber bishmecha hayra la'am hazeh
The Midrash gives a mashal ofto a king whose subjects acted disrespectfully to him and incurred his wrath. The king's faithful minister, seeing how angry he was at the people, went and slighted the king's honor in some small manner. He went to the king and confronted him, saying, "if you're angry at the people, you should be angry at me, too." The king had no choice but to forgive all parties.
Moshe Rabeinu saw that the RBSO was angry at the yidden for not believing Moshe and for complaining so he went and joined them in a way, in order to placate the RBSO (keveyachol) by doing so.
The RBSO told Moshe "ata sir'eh", "now you will see" what I will do to Mitzrayim. Rashi says that the meaning is that now you will see, and not the war of the 31 kings which will take place when the yidden enter Eretz Yisrael. This is usually understood as a punishment to Moshe Rabeinu. Perhaps it can be understood somewhat differently.
Chazal say that Moshe Rabeinu is buried in the midbar and not in Eretz Yisrael because he will lead the Dor Hamidbar into Eretz Yisrael when Moshiach comes.
So what the RBSO is saying may be this: You stood in solidarity with the yidden when they were down and out. I am giving you another opportunity to do so, when the Dor Hamidbar will be forced to remain in the desert and not be allowed into Eretz Yisrael you will remain with them too and ultimately lead them into Eretz Yisrael.