It's not a fact that the US needs to spend 50% of the global military budget. If anything is myopic, that is.
It can still be argued that it's worth the expenditure. But the cost should be weighted against the alternatives. Neither Russia or China can be said to threaten the US with their military, except with nuclear weapons, but no defence budget protects against those.
I'm saying that China was extremely prosperous and growing from 1980 to the present day, without increasing the size of its army 10x-20x-or-however-much-they're-increasing-it right now.
They don't have a real need for a bigger army, they could for sure defend themselves against any neighbor, even India and Russia, with "just" the military from 20 years ago.
If there was a total war scenario with Russia and China, all bets would be off regarding U.S. military budget, as they would most certainly be a dire threat to numerous allies of the U.S.
In practice, nothing (against multiple multi-warhead ICBMs). If your foe only has one or two ICBMs maybe you have even odds. Against any nuclear-armed nation other than present-day North Korea, nothing. Even if you could intercept 90% of missiles/warheads if the opponent fires 20 (or 2000), you're screwed.
There are multiple stages of flight in which one can attempt interception, but only the final stage after separation of warheads is realistic (which multiples the number of projectiles to intercept greatly): you can't intercept at accent unless you're very close to the launch site, hitting stuff in space with directed energy is defeated by a simple mirror, and hitting it with an inceptor is not feasible.
It can still be argued that it's worth the expenditure. But the cost should be weighted against the alternatives. Neither Russia or China can be said to threaten the US with their military, except with nuclear weapons, but no defence budget protects against those.