I mean, it's not a bad theory. The wikipedia article on neutrons discusses how this was an assumption somewhat like this about 100 years ago ("nuclear electrons") and the lines of reasoning that led to abandonment of the idea: https://en.wikipedia.org/wiki/Neutron#Discovery -- it has to do with the observable quantity called "spin"; the spin of a neutron is not the sum of the spin of an electron and a proton, so it must be something else.
