It's significantly weakening one of the US's greatest geopolitical enemies for a small fraction of the US defense budget.
Even if you don't care about the US's geopolitical aims and want to reduce the defense budget, eliminating Russia as a military threat is the best justification for reducing the defense budget and stabilizing European democracy.
Russia is not a military threat though. It has an economy the size of Italy. Three US states each have bigger economies than Russia. The US blew past it on every level militarily, technologically, and economically decades ago.
Even if you don't care about the US's geopolitical aims and want to reduce the defense budget, eliminating Russia as a military threat is the best justification for reducing the defense budget and stabilizing European democracy.