Germany doesn't share a border with Russia. Even if Russia based an attack out of tiny Kalingrad, Russia would first need to pass through Poland. To get to Germany from the Russian mainland, they'd need to pass through both Poland and one or more other intervening nations.
Anyway, German spending is adequate for the current threat. This isn't like the days of the Soviet Union, with the Soviet Union spending well over the combined military budgets of NATO's European members. Nor is it like the bad old days when the Soviets and their Warsaw Pact allies were literally bordering directly on countries like Germany. These days, Russia spends relatively little on its military and would have to fight through hundreds of miles of hostile territory even to reach a nation like Germany.
Using IISS numbers, Russia spends $65.6 billion on its military. That's a bit more than Germany, but considerably less than the combined spending of Germany and France, and vastly less than the 28 EU member states. Any number of combinations of Germany and a few European allies would be more than Russia could handle in a conventional war. For example, just the European countries speaking Germanic languages spend around $144 billion on their militaries, giving them more than a two-to-one advantage over Russia if you were to picture a war along those lines. Unless you can picture all those Germanic countries, as well as France and Italy, standing by and letting the Russians roll into Berlin, the alliance has more than enough firepower to keep the Russians in check, without the need for the US bases there.
That said, I'd welcome the US pulling its military bases out of Germany. It would be a temporary hit to local German economies in places where that employment vanished, but they'd also get back some valuable land to develop, some of it near urban areas where land is at a premium, so long-term they'd be better off. Meanwhile, it would make it harder for the US to run offensive military operations without first convincing its allies that they're actually needed. That would spare us some of our more half-baked interventions. Currently we lean on those bases in places like Germany to administer our wars in the Middle East, and because they're treated as US soil rather than allied soil, we can do so even in cases where our allies disagree with our war (as Germany did when we decided to conquer Iraq). I like the idea of moving to a situation where we need to take the concerns of our allies more seriously -- if we'd listened to Germany's objections more back in 2003, it might have saved us thousands of American lives, trillions of American taxpayer dollars, and untold losses of America's international political capital.