Why, though? He could have been warning them of the impending destruction (which is actually what the book does do - it never refers to it as an event that has happened). What you're actually saying is that Jesus is much less likely to have predicted it, and hence Matthew written his warnings, than Matthew was to have known it happened and written to explain why.
But you are simply describing the narrative style of the gospels - effectively narrating the claimed story of Jesus as if it is contemporaneous (i.e in the present) when we know that they were written at best decades later and, in reality, discussing something in the past.
So, of course, Matthew needs to allude to the destruction rather than simply say 'hey look what happened 5 years ago in CE70'. But also prophesying is a highly risky game unless you are writing with hindsight - in other words the thing you are prophesying has already happened, which is, of course, what Matthew is doing. Knowing full well that the destruction has happened but writing as if it is some kind of prophetic future event.
So basically what Matthew is doing is creating a narrative as follows: 'Judas rejected and betrayed Jesus and look what happened to him - he was punished and cursed. And now the whole of the jewish people are rejecting Jesus and so they will be punished and cursed' - knowing full well that this had already come to pass through the destruction and the narrative that the early church were creating around that event.
So yes, it only really makes sense if written after the destruction (as basically all serious bible scholars agree).
And I come back to my other point - the earliest actual texts we have on field of blood or prophecy of destruction are from way, way later than CE70 so even if there was some text earlier than the destruction there was plenty of opportunity for this to be revised to take account of the destruction and the purported fulfilled prophecy (actually merely fitting a prophetic narrative to an event that had already happened).