People with reduced immune function benefit from reduced disease prevalence, it's pretty straightforward.
For example it's not great that some older folks will inevitably die of COVID over the next 20 years, but it's better than just letting happen over the next 12 months.
Those people are already getting their second or third boosters. What makes you think it would take 20 years for them to get it instead of inside of a year? Covid is highly infectious and spreads easily in spite of mass vaccinations. Getting vaccinated and boosted up is all you can do and even then if you're in a high risk demographic you can still die.
For example it's not great that some older folks will inevitably die of COVID over the next 20 years, but it's better than just letting happen over the next 12 months.