If stealing information were the key, Russia would have the most powerful military on earth, with a combination of their espionage of US military technology over 60 years and their very substantial defense budget.
It's drastically more difficult than that.
The US military also doesn't spend trillions on R&D - R&D is the light end of their budget ($80 billion per year for R&D, testing, evaluation). Operations, maintenance, and personnel are the principle costs of the US military budget.
You're talking about China (or Russia etc) stealing a fraction of that $80 billion total. The amount of that theft they can then actually put to use, is a further reduced fraction.
It's drastically more difficult than that.
The US military also doesn't spend trillions on R&D - R&D is the light end of their budget ($80 billion per year for R&D, testing, evaluation). Operations, maintenance, and personnel are the principle costs of the US military budget.
You're talking about China (or Russia etc) stealing a fraction of that $80 billion total. The amount of that theft they can then actually put to use, is a further reduced fraction.