Linda asked to go on permanent Soma vacation when she returned to the Brave New World, because she wouldn't have to suffer the headaches and vomiting that accompanied other forms of escape. She asked that she go on a continuous Soma holiday because she never again wanted to face the fact that she was now an imperfect person in a perfect world.
Dr. Shaw was against the idea at first, knowing that the Soma coma would eventually kill Linda by paralyzing her respiratory system and stopping her breathing.
"[The Soma holiday] will finish her off in a month or two," said Shaw, "One day the respiratory centre will be paralyzed. No more breathing. Finished. And a good thing too. If we could rejuvenate, of course it would be different. But we can't."
Since there was no hope in making Linda a perfect person again, Shaw agreed to Linda's permanent holiday request. When John asked if that would shorten Linda's life, Shaw equivocated.
While he agreed the overdose of Soma would shorten her physical life, he said that the holiday was like eternity...she would spend the rest of her life in complete bliss and free from pain and suffering... like a paradise out of time.
So, while her body would die sooner, her bliss would be longer than expected, which was a good trade off.