For several reasons:
1) It's not just 70-year olds that end up in hospital if unvaccinated. Omicron, for example, seems to affect people regardless of age;
2) OK - so what? - you claim to point 1. Well, if enough people aren't resistant, hospitals are swamped - and so people who should be hospitalized are not. And among deathly ill individuals who cannot be hospitalized due to lack of capacity, the death rate is extremely high - increasing the overall death rate from rampant COVID infection to much higher than 1-in-100;
3) Moreover - who foots the hospitalization bill? Society is far better off if unnecessary hospitalization is avoided;
4) Long term effects of COVID infection aren't well understood - but growing evidence suggests that some people exposed to the virus - even those who never needed hospitalization - suffer long term, severe, adverse health effects - these people are termed "long haulers."; what is the cost to society for such long hauler individuals who would likely not be in such circumstance should they have been vaccinated;
5) Massive expense of hospitalization aside - there are only so many hospital beds - once filled, hospitals are swamped and the mortality rate rises dramatically;
6) COVID attacks numerous of cells/organs. Survive COIVD - great! But what damage has it caused meantime? (see 4 above)
===
Expert thinking - vaccinate, wear masks, practice social distancing. These are the best ways to minimize the impact of COVID.
I'm going with the experts.