Indeed, let us not forget that Alexander wanted to keep going east, but his troops had had enough. This was even after the victory at the Hydaspes River. Had they been willing to continue, I say Alexander could have made it at least as far as the Ganges, if not all the way to China. Another thing we must remember is that at the time, India was immensely fractious, while Alexander's Macedonians had better armor and tactics, as well as superior numbers. Indeed, the only thing stopping them from reaching China would be disease and a lack of knowledge of the terrain. Of course, they could then easily pull replacements from Persia, and the Persians had never been on very good terms with India anyway, from what I've heard.