I don't think that's a given at all actually. Look how Yemen was able to defeat US navy in the Red Sea with the weapons supplied by Iran. Now imagine that Russia starts sending advanced weapons to Iran and Syria. US and Israel won't be able to bomb Syria anymore. Iran will start using modern jets like Su35, etc. Russia is also actively collaborating with countries in Africa to clean up US backed terrorists and push out AFRICOM. Also, consider Russia helping strengthen DPRK against the regime in the south. This will put a lot of pressure on US resources in the region. Same goes for Russian military cooperation with China.

Russia doesn't need to start a war with US, simply to make it increasingly expensive for US to maintain its global military presence. US is far more extended than Russia in this regard. So, Russia can have an asymmetric response to what US is doing in Ukraine that's going to be just as damaging.