If you have to add 30.000 soldiers a month to keep the war going, and you succeed in doing that, then you can continue.

But if suddenly you need 50.000 (the Ukrainian goal) and you start to struggle to find "even" 30.000 a month, then your war is basically lost.

Not instantly, but it's like driving to another city and there is no gas station on the road, and you can do it at 90km/h because the car consumes just that much. If you start going 130km/h and your car now consumes more, it's not an instant catastrophy, but everyone in the car now knows you won't make it, and might take action too.