Say if you add NH3 to a solution, and it accepts a proton, it still becomes NH4+ in a solution, which is also acidic. A base should decrease the hydrogen ion concentration in a solution, but how does a solution become more basic if it now has an increased concentration of NH4+?
Severe misconception. Hydrolysis reaction is NH3 + H2O --> NH4+ + OH-
Hence, dissolving NH3 in water produces OH- ions, making the solution more basic, as [OH-] > [H+]