Let’s all take a step back from this passage. There’s a bad question and good question to ask at this point.
The bad question is, “Why does God urge Ahaz to trust Him by prophesying about a future event rather than prophesying about something dealing with Ahaz’s most immediate circumstances (the army coming against him)?” This is a bad question because it implies that the future virgin born Messiah has nothing to do with Ahaz’ current problem. The good question is this, “How does this sign of a promised future Redeemer encourage Ahaz to trust God in the present moment?”
Here’s what I think God is up to: though the promise of a future Redeemer does not remove the imminent threat facing Ahaz and Judah, it does allow Ahaz and Judah to encounter their current problem with hope that one day God will right this wrong that’s coming upon them (if they believe it).
God is saying to Ahaz and Judah, “I have given you My sign, the virgin shall give birth to My Son, and He will save His people from their sins, He will make all things right, and He will usher in a new age and a new kingdom into the world. Trust in this sign Ahaz, trust in this sign Judah. Trust that I will one day make this right! Trust that I’m in control, no nation is! I know that being still and trusting Me looks and feels like the most unpractical thing to do right now Ahaz, but you need to trust Me, and not run to another source to save yourself and your people.” We can read how the events unfold and 2 Kings makes it clear that Ahaz did not trust the sign God provided, but God was faithful (as He said He would be through Isaiah’s son Shear-Jashub!) and kept a remnant of the people of Israel alive.