I think I had read something to the effect that those who had recovered from COVID and get vaccinated generate a robust immunity. Maybe this which seems very conjectural at this point:

[nytimes.com]

““People who were infected and get vaccinated really have a terrific response, a terrific set of antibodies, because they continue to evolve their antibodies,” Dr. Nussenzweig said. “I expect that they will last for a long time.”

The result may not apply to protection derived from vaccines alone, because immune memory is likely to be organized differently after immunization, compared with that following natural infection.
That means people who have not had Covid-19 and have been immunized may eventually need a booster shot, Dr. Nussenzweig said. “That’s the kind of thing that we will know very, very soon,” he said.”

Vaccinations may ameliorate long-hauler issues somewhat.

[aarp.org]

Kids may in general have less issues with COVID but some do develop complications and bringing down the age range of the vaccinated may help with reducing numbers of people who suffer and also who spread.

My second mRNA shot kicked my butt for a day or so, but it beat the potential alternative of suffering actual COVID. Any actual risk of the vaccine seemed acceptably low to me and was outweighed by the sense of relief from anxiety I had for the past year. Still seems surreal.

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.