If they conquer Ukraine, they then have Ukraine's resources, technical capabilities, and a fresh group of young people to conscript into service.

We shouldn't be scared of Russia, per se, they would be easy to defeat if we bothered to try rather than if we tried to drag out this war as long as possible to try to weaken Russia. But if we let Russia win, they will rebuild far stronger and take over the next country, and grow stronger. And again, and again.