Matthew 24:29 is at the end of the great tribulation. Matthew 24:38 describes what what the world will be like before the great tribulation. Jesus coming for the resurrection/rapture will be before the great tribulation begins.
Matthew 24 :39 is the flood, which like the great tribulation, is a time of mass dying. The eating and drinking, marrying and giving in marrage in Matthew 24:38 is before a time of mass dying.
Matthew 24:
37 But as the days of Noe were, so shall also the coming of the Son of man be.
38 For as in the days that were before the flood they were eating and drinking, marrying and giving in marriage, until the ay that Noe entered into the ark,
39 And knew not until the flood came, and took them all away; so shall also the coming of the Son of man be.
42 Watch therefore: for ye know not what hour your Lord doth come.
All of that is absurd, the fact once Jesus gets to verse 29 He never focus' on great tribulation again. His focus from that point on is what follows after great tribulation has ended. You have Him talking about great tribulation in verses 15-26, then have Him talking about His coming after great tribulation in verses 30-31, then have Him once again talking about verses 15-26 in verse 39. Thus you have Him being all over the place rather than having Him being systematically coherant about things instead.
The context surrounding verse 39 is not great tribulation, the context is judgment that follows after great tribulation.
Assuming you are still alive at the time, it's only a matter of time before you are undeniably proved wrong. The fact you are still going to be here when verse 21 takes place whether you like it or not. There's only 1 way that you won't be here during when verse 21 takes place. And that is, if you die before that verse is fulfilled.
Granted, Preterists and those that agree with Preterist thinking but are not Preterists themselves, would have us believe that verse 21 has already been fulfilled 2k years ago. The same goes for these. It's only a matter of time before they are proved wrong as well, when verse 21 begins taking place and that they can no longer deny it since they would be living right in the midst of it.