This is exactly the problem. Tyrande and Elune seemed to have made a pact, something Tyrande promised to the people of Telldrasil. Tyrande, indirectly or not, was willing to give her life to see Slyvanas brought to justice, to kill Slyvanas for what she did.
In the cinematic, Tyrande asked Elune for the power to kill Slyvanas, quite literally saying “my life for hers.”
Elune granting her the power to go Super Sayian was basically Elune accepting that trade, otherwise why give her the power to fly in the first place?
The cinematic could have been done better, with Tyrande on the verge of killing Slyvanas, with her 9 Valkyrie teleporting in and chaining back Tyrande.
Ysera then flies in and breaks the chains, freeing Tyrande, but Slyvanas still gets away.
Now you don’t have this wishy washy goddess who just randomly gives power, but strips it away 3 seconds later. One who has already shown she has the power to give to explode an Old God, even at the cost of said person’s life.
(Aman’thul ripping out an Old God from Azeroth, or Sargeras Splitting a planet in half, unless she isn’t a Pantheon, she should have the strength.)
The Night Elves would be heart broken, as would Malfurion, but that was the price Tyrande, and in respect, the Night Elves, were willing to pay.
Now what happens? Tyrande and the Night Elves lose all hope because their goddess not only abandon them during the Burning of Telldrasil, but now also is allowing this mass murder to get away with it.
When the goddess of the Moon has continuously abandon you, what left is there for the Night Elves?