Good that it went unimpeded for a long time then.The balloon is almost certainly a SIGINT platform, so every second it is left in the air is time that the US is allowing it to gather more critical data, hence the urgency to shoot it down. Destroying it is necessary as a defensive measure.
And I don't think the US needs to justify shooting it down despite the military necessity to do so. It is an adversarial military platform operating in the US' national airspace for crying out loud.
Also when you're an aggressor state that sends your manned planes into Chinese airspace explicitly to forment civil war and expand your territory, you can expect to get spied on.
Yeah, if there are USA unmanned platforms in Chinese airspace, China will shoot them down. And at the same time China will work towards unmanned platforms surviving as long as possible while monitoring US aggressor activities. That's called double standards, cry ab it.