What theory does the Time Traveler reach about the people of the past and their hard lives verses the people of the future and their easy lives?
The Time Traveller expects to find a civilization that is far advanced beyond his own time when he arrives in the future; however, what he finds is quite the opposite. He is disgusted by the Eloi's laziness and intellectual limitations and wonders where humanity could have gone wrong to cause people to devolve in such a way.
He first concludes that COMMUNISM is to blame, since it would eliminate competition, without which there would be no need for invention. But after discovering the Morlocks he settles on a new theory. CAPITOLISM is to blame for the degradation of mankind. The Eloi developed as weaklings in every way because, as upper class, they had no need to work their bodies or their minds. The Morlocks became more like apes and very physically agile because their social position dictated that they evolve with those qualities in order to survive in their initially subservient role. Ironically, the lower class Morlocks became stronger than their overlords because they were the ones who had necessity. They had to maintain some technology in order to survive.
So, I guess the point Wells was making is that necessity breeds excellence, and luxury breeds weakness.
I think you might be asking what conclusion the Time Traveler comes to regarding people's lives in the past as compared to the future. Inventions such as electricity and the telephone made people's liver easier and caused a changed in how people viewed their environment. They wanted the ease that technology offered and promised. Instead of their lives becoming better, however, the Time Machine showed a future that wasn't promising. People were controlled by technology rather than the other way around. Humanity evolved into two different classes, the workers and the owners.
Other writers also began to write about the dangers of technology on society after Wells wrote about evolution and natural selection. Few of them felt technology would be an asset to our lives. I wonder what those writers would think of our lives today with computers and cell phones?
It is less a theory that the time traveler makes than one that Wells applies and capitalizes on: evolution. Wells was born not long after Darwin published his theories on evolution, and England was still in the throes of dealing with the implications of those theories. Rather than being descended from the angels (and relatively static) humanity was heir to the apes, and likely to change in response to future environmental changes. In the Eloi and the Morlocks, Wells gives the readers a socialist critique of the division of society into classes by extending it into the future, when inbreeding within each group makes the social distinction into a biological one.