I am a Rapturist and I believe in multiple catchings away of certain of God's people, not just the usually emphasized one of the body of believers before times of wrath. And, I'm also a prophecy student and believer, not relying on any one isolated verse for interpretation but on a very large mosaic of scriptures linked by the Rules of Hillel that I may get into later on. And, I also believe in the Resurrection, and the grounding reasons for my believing it are from the study Dr. Gene Scott made of it. So those can be taken as exemplars of the frame of reference through which I see Mark 9:1.
In discussing Mark 8, Matthew 16, and Luke 9, I think it is valid to presume that after Jesus talked about his death and resurrection, that he would then talk about what comes after his resurrection. His speech goes from talking about the suffering of the son of man, then being killed, then being raised after three days. Nothing about the ascension per se. Then, after an interruption by Peter, Jesus talks about what faithers will go through -- the faith response to the resurrection, which is what the entire church age deals with, quoting part of it from Mark 8:34-5 in the KJV here:
"And when he had called the people [unto him] with his disciples also, he said unto them, Whosoever will come after me, let him deny himself, and take up his cross, and follow me. For whosoever will save his life shall lose it; but whosoever shall lose his life for my sake and the gospel's, the same shall save it."
And then he moves into this speech in the KJV:
Mark 8:38: "Whosoever therefore shall be ashamed of me and of my words in this adulterous and sinful generation; of him also shall the Son of man be ashamed, when he cometh [is established] in [with, by means of, through] the glory of his Father with the holy angels." [Show me where that happened at the Ascension? Looks to me like He was LEAVING.]
So here we're already past the entire history of the church age at the end of days, he's already talking about his coming back in glory to establish his kingdom, setting up judgment, and makes a parenthetical note about the people of God escaping and establishing the kingdom in that time in Mark 9:1 (retranslated by me):
"And he said to them, Amen (trust in this), I tell you that there be a time when they have established (have escaped) under these circumstances: Whosoever may not taste of death for themselves, till they may see God's kingdom has come (been established) through (the) power of hosts."
And there the speech ends -- at the end.
Since the ascension is actually not mentioned in Mark 8 (or Luke 9, or Matthew 16; I dare you to try to find it there), it's just not the point of this speech. I think the end of the speech continues the thoughts begun in the previous verse about the coming of the son of man in his Father's glory with the angels, which students of Daniel and Revelation may take as the last days, when wrath comes on the world as God overthrows the world and sets up (establishes) His own kingdom. So, I believe the escape clause is written in 9:1 for people of that future time to escape that wrath, not because Jesus was so thrilled about the Ascension that He was daring people to show up for it -- we're talking about someone who walked on water; a little levitation and catching the divine equivalent of a taxi is nothing to the guy. The bigger events by far were the ones he did forecast in these passages -- his suffering, death, and resurrection after three days, followed by what he told faithers what their attitude needs to become -- everything the church is supposed to teach in a compressed nutshell. So what Jesus went to next in subject matter was the end of the church age and His second coming, including His kingdom coming in power with the angels.