Your reading of this is rather extreme, and I feel you've missed the point by thinking that Russell is advocating to avoid fun intentionally at times so as to preserve a happy life. I read it as more an observation that the boring times, those in which there's a lack of stimulation, are an important part of a balanced, healthy life.
A more modern take on his advice would be to avoid mindlessly surfing the internet every dull moment in which we are waiting a few minutes for some time to pass. If you haven't taken an extended, conscious break from the internet outside of your work needs (no social media, no news, etc), I recommend it. It was very enlightening for me to feel this lack of stimulation. I certainly enjoy my time on the internet when I'm there because I want to be there, not because I'm just looking for something to kill the time.
Similarly, I've stopped listening to music while coding because I realized that the listening without mindfulness was killing my passion. After doing so, the passion has been restored.
His words, too, remind me of the drug user's plight. My sister works in drug rehabilitation and has told me about the struggles many go through known as Post Acute Withdrawal Syndrome. From my naive understanding, these people have adapted to the constant, unnatural amount of dopamine flowing through the brains during addiction by producing more dopamine receptors. When the drug is gone, those receptors are still there. But, the body is not capable of naturally releasing the dopamine required to fill these receptors even during the highest of natural moments, leading these people to experience serious anhedonia -- an intense lack of pleasure from everyday life. It takes a long time for these receptors to downregulate, which is the reason for such a high rate of relapse for certain drug types.
I think of Emerson's words a bit: "If the stars should appear one night in a thousand years, how would men believe and adore; and preserve for many generations the remembrance of the city of God which had been shown!"