I think the part in this that many people are overlooking is mutation. If we can limit via vaccination the spread, we also limit the chance for new strains to develop. The more the virus mutates, the greater the chance that our antibodies will be insufficient to protect against serious illness. Some viruses are more stable and don't mutate, but SARS-CoV-2 is far more volatile and has mutated significantly in a relatively short time. So far, new strains have been effectively warded off by the immunity from existing vaccines and prior illness, but some strains are proving to be more virulent and are being watched more closely for signs of antibody resistance. In particular, there's a "delta" strain that's spreading quickly through young people in Europe that has immunologists concerned. I heard about this only this morning, so I haven't read up on the details yet. Couple this propensity for mutation with the likely temporary nature of immunity from vaccination or prior infection, as @Larimar pointed out, and speed of reaching widespread immunity is a top priority. Although children under the age of 18 are largely asymptomatic and survivability is very high for that age group, that is the most likely group to contract and spread the disease — especially in countries where social distancing measures are being relaxed — and it is likely that viral mutations will appear more often among school-age children. My hope is that, even with mutation, we've slowed the spread enough and increased medical care and capacity enough that our ability to effectively treat infection will be sufficient to keep the death toll lower. Some immunologists have speculated that COVID-19 may mutate eventually to be no more severe as other coronaviruses, to be relegated to one of the many common cold viruses — but there's no guarantee and we're not there yet.

You are not taking into account that not all disease is a one and done. This one...my immunity was gone after six months. Getting the first shot was like getting covid again, except it only lasted four days instead of ten. So your premise is incorrect.