Let's not ignore the years before LLM-oriented data centers actually if ever switch to renewable energy, never mind whether the switch will "reverse" the long-term effects of high pollution during the first years. Refining GP's and your analogies:
Imagine if an overweight person could only lose significant weight if they temporarily underwent obesity for a few years. While many overweight people exist, we should actively prevent more people from becoming overweight, and we should change the socioeconomic and political circumstances that contribute to excessive weight gain.
Imagine if a moderate exercise session causes a person to feel severe hunger pains for a few hours that do not immediately abate upon eating. Such an unfair biological condition would incentivize the person to avoid moderate or vigorous exercise. If the person attempts to power through the pain, the pain may trick them into eating more than they need to at the expense of their wallets, their health, and collective food supply. We should encourage such individuals to set aside longer times for light exercise if their finances and work allow them to, teach people to reduce their extra calorie margins without sacrificing necessary nutrients (especially because taking in less extra most-things is easier than removing extra most-things), and research palliative and curative medical treatments for this condition.