On what basis, precisely, can you claim that this was a vision of the future, because you haven’t given one?
Looking at the context:
Mat 17:1 And after six days Jesus took with him Peter and James, and John his brother, and led them up a high mountain by themselves.
Mat 17:2 And he was transfigured before them, and his face shone like the sun, and his clothes became white as light.
Mat 17:3 And behold, there appeared to them Moses and Elijah, talking with him.
Mat 17:4 And Peter said to Jesus, "Lord, it is good that we are here. If you wish, I will make three tents here, one for you and one for Moses and one for Elijah."
Mat 17:5 He was still speaking when, behold, a bright cloud overshadowed them, and a voice from the cloud said, "This is my beloved Son, with whom I am well pleased; listen to him."
Mat 17:6 When the disciples heard this, they fell on their faces and were terrified.
Mat 17:7 But Jesus came and touched them, saying, "Rise, and have no fear."
Mat 17:8 And when they lifted up their eyes, they saw no one but Jesus only.
Mat 17:9 And as they were coming down the mountain, Jesus commanded them, "Tell no one the vision, until the Son of Man is raised from the dead." (ESV)
Everything there happened at that point in time. Jesus’s telling the disciples not to say anything about it all until his resurrection in no way whatsoever means that the event was speaking of something future. That would make no sense.
We also have the following:
Mat 22:31 And as for the resurrection of the dead, have you not read what was said to you by God:
Mat 22:32 'I am the God of Abraham, and the God of Isaac, and the God of Jacob'? He is not God of the dead, but of the living." (ESV)
That last statement cannot be true if Abraham, Issac, and Jacob were not alive.