I'm not sure what you mean that the necessary studies have not been done. Because they have and broadly agree that a fourth vaccination provides benefits against hospitalization (though not infection) for older people, generally based on epidemiological measures of population-wide severe cases of COVID-19, not immune response.

Bar-On YM, Goldberg Y, Mandel M, Bodenheimer O, Amir O, Freedman L et al. Protection by a Fourth Dose of BNT162b2 against Omicron in Israel. New England Journal of Medicine. 2022. doi: 10.1056/NEJMoa2201570.

Magen O, Waxman JG, Makov-Assif M, Vered R, Dicker D, Hernán MA et al. Fourth Dose of BNT162b2 mRNA Covid-19 Vaccine in a Nationwide Setting. N Engl J Med. 2022. doi: 10.1056/NEJMoa2201688.

Gazit S, Saciuk Y, Perez G, Peretz A, Pitzer VE, Patalon T. Relative Effectiveness of Four Doses Compared to Three Dose of the BNT162b2 Vaccine in Israel. medRxiv. 2022:2022.03.24.22272835. doi: 10.1101/2022.03.24.22272835.

Arbel R, Sergienko R, Friger M, Peretz A, Beckenstein T, Yaron S et al. Effectiveness of a second BNT162b2 booster vaccine against hospitalization and death from COVID-19 in adults aged over 60 years. Nature Medicine. 2022;28:1486–90. doi: 10.1038/s41591-022-01832-0.

Regev-Yochay G, Gonen T, Gilboa M, Mandelboim M, Indenbaum V, Amit S et al. Efficacy of a Fourth Dose of Covid-19 mRNA Vaccine against Omicron. New England Journal of Medicine. 2022;386:1377-80. doi: 10.1056/NEJMc2202542.

Muhsen K, Maimon N, Mizrahi AY, Boltyansky B, Bodenheimer O, Diamant ZH et al. Association of Receipt of the Fourth BNT162b2 Dose With Omicron Infection and COVID-19 Hospitalizations Among Residents of Long-term Care Facilities. JAMA Internal Medicine. 2022. doi: 10.1001/jamainternmed.2022.2658.

Amir O, Goldberg Y, Mandel M, Bar-On YM, Bodenheimer O, Freedman L et al. Protection against omicron severe disease 0-7 months after BNT162b2 booster. medRxiv. 2022:2022.05.04.22274647. doi: 10.1101/2022.05.04.22274647.

Cohen MJ, Oster Y, Moses AE, Spitzer A, Benenson S, Israeli-Hospitals 4th Vaccine Working G. Association of Receiving a Fourth Dose of the BNT162b Vaccine With SARS-CoV-2 Infection Among Health Care Workers in Israel. JAMA Network Open. 2022;5:e2224657-e. doi: 10.1001/jamanetworkopen.2022.24657.

Grewal R, Kitchen SA, Nguyen L, Buchan SA, Wilson SE, Costa AP et al. Effectiveness of a Fourth Dose of COVID-19 Vaccine among Long-Term Care Residents in Ontario, Canada. medRxiv. 2022:2022.04.15.22273846. doi: 10.1101/2022.04.15.22273846.