The meek who will inherit are those who live by the faith and conviction that the things that happen on this Earth, or the material realm are not going to go on forever.
It is only a threat to those who act on the folly of their own desires and narrow understanding. I think to be certain it is a caveat. It is a call to act with a degree of compassion towards “the least of these” and to realize that all you do will eventually reap its fruit, or outcome, for better or for worse. You do not have to let others “step on you” to refrain from action towards someone who has offended you. I think that it is a passage that purposefully calls people to refrain from action simply because the actions that are immediately available to them will be forthcoming in anger or some form of passionate outburst. It is better to be self controlled than to act rashly and have the outcome of your actions be worse than the initial offense was.
In conclusion, it is a passage about restraint from action, an active restraint that requires faith and humility. If you believe that you are entitled to impute a judgment on others because of an offense proffered unto you than you will also have to deal with the endless back and forth, or unforeseen repercussions of your actions.