A more balanced view, in my opinion, is that (by increasing our power) science and technology have exposed us to temptations that earlier generations did not have to worry about. If we rise to the challenge and overcome these temptations, our chances for happiness are greater than those of previous generations. If we succumb to the temptations, we will be less happy.
The key metaphor here is the story [page 51] of aboriginal peoples who drink themselves to death once the white man makes alcohol easily available to them. Because we can satisfy our basic needs and generate excitement and entertainment easily, we modern people are open to the following addictive cycle: satiation leading to boredom leading to easy but superficial titillation leading back to satiation. Nothing in this cycle leads to real happiness, but it is difficult to break out of.
A similar temptation involves work. Moralists in an agricultural society can fulminate about the evils of idleness without harmful effect, since night and winter guarantee everyone sufficient rest and leisure. Today, however, we are confronted with the temptation to prove our "virtue" by working 24 hours a day, 365 days a year. When we fail to do this, we may feel guilty. We need to balance work and leisure consciously, and this requires us to be wiser than our ancestors. If we are not wiser, we fall prey to an unhappiness that circumstances once rendered impossible.
Now take a look at theme C and notice this: If the modern world is worse than the ancient world, then a theory of hope that depends on the progress of humankind falls apart. Russell had a vested interest in seeing the modern world in a positive light.