"Leaky vaccines" result in catastrophic situations like Marek's disease.
Against the original Alpha perhaps. However Delta effectiveness is nowhere near 90%...this is precisely the reason for the push for boosters.
[1] https://www.nejm.org/doi/full/10.1056/NEJMoa2108891
[edit] I'm sure with more data that number might change up or down a little, but it seems in line with other findings.
We don't have precise data about the required level of antibodies to stay healthy yet and in quite a few countries the infection rates are currently high enough to pose a real risk for vaccinated patients in the vulnerable group. The safe play is to give booster shots to keep the immune response at its best.