We have to protect the vaccine too to protect ourselves which means not letting it run away. I don't think it will anyway.
It's a difficult argument and I am not totally convinced by it myself. You can't say if we have 250,000 more cases, the vaccine effectiveness will fall by x. It's all rather nebulous.
We should be cautious though as we are so reliant on our immunity continuing to work. We're almost there now I feel. In those countries where there is no Indian variant or it's very very low, infections have gone right down (Israel, USA).The Kent variant is also goint to be extinct soon so if it hadn't have jumped we'd be done. But of course it is a virus, and it mutates.