How can we trust scientific theories like gravity or evolution when they depend on assumptions that might change?

Is Trump right given that America has lost over thousands of manufacturers and unable to fix its infrastructures because of Asian countries’ unfair trading practices?
My avoidant lady I was dating (with 2 Phantom X’s I would add) discarded me after 6 months last weekend. I don’t think I should hold my breath waiting to hear from her again. What are your thoughts?