For example. If there are storms in the area and that folks are being told to be on the look out for tornadoes, but that a tornado never occurs, were these folks watching in vain, being told in vain to do something that never happens to them? Probably not, right?
The point being, just because someone is being told to be watchful about things, but that these things never come to pass, that doesn't mean they are being watchful in vain, it just means they are being watchful just in case.
Unless there are no more disciples in any sense after that of the days Christ and His main disciples at the time lived during, and unless the body of Christ is not essentially one big family, this means what He says to one disciple can apply to other disciples, even to disciples not even born yet if what He indicated to be watchful about, that none of those things ever came to pass in their lifetimes. Meaning during the days of His main disciples at the time.
Obviously, some things can only apply to those living at the time, such as what eventually happened in 70 AD. That is a one time event that will never be repeated. Yet, some things can apply to both His disciples at the time and to future disciples.
But when Jesus is telling His disciples at the time, for example, to be watchful for His coming, but that His coming never happens in their lifetimes, obviously this means that disciples in the future need to be watchful as well, because the odds are that eventually He is going to come during someone's lifetime, but if no one is being watchful at the time, because some are assuming Jesus was only meaning anyone alive during His day and time, we then encounter a problem. These disciples are not doing what Jesus said to do, to be watchful of His coming. I'm assuming this would mainly only be meaning Preterists since they are mainly the ones who a lot of the time are interpreting much of what Jesus said, in a vacuum.