A reduction in something bad is still a good thing, and to be welcome, even if it falls short of an eradication, isn't it? To strain my car crash analogy further, drink-driving laws clearly wouldn't eradicate all car accidents even if everyone adhered to them at all times, but you're still far less likely to have an accident if you're sober vs. if you're wasted.
And a complete prevention of the transmission of a highly infectious variant would not be expected when only 60% of the population has been vaccinated, so saying "people are still catching it, therefore the vaccine is bunk" is a strawman argument.