If Jesus' coming was still thousands of years away, His statement in John 21:22 makes no sense in context:

"If I want him to remain until I come, what is that to you?"

This clearly implies that His coming was a real possibility within John's lifetime. Otherwise, it's meaningless speculation or misdirection. Why even say it?

Jesus had already told His disciples:

"There are some of those who are standing here who will not taste death until they see the Son of Man coming in His kingdom" (Matthew 16:28).

"This generation will not pass away until all these things take place" (Matthew 24:34).

If we believe Jesus was consistent in His message, then John 21:22 is just another reinforcement of the soon-coming, within-their-generation fulfillment. And John did remain alive until that coming, not a global, visible return in the clouds, but the covenantal judgment on Jerusalem in AD 70, the same event Jesus described in Matthew 24.

So the question isn't just, "Why would Jesus say this?"

It's also, "Why do futurists twist this clear time statement into something completely unrecognizable from the original context?"

If His coming was thousands of years off, the statement is misleading.

If it was near, as Preterists affirm, it makes perfect sense.