Most religions require an absolute distinction between animals and people. A common divide is that people go to heaven, other animals do not. Even for those of us who have no religious inclinations it still comes out as one of the most important divisions. No matter how highly we think of some species, we would never give them the right vote simply because we know they couldn’t use it. Sapience is not all about having extra privileges and rights – its also about obligations. It is pleasing to see that so many recognise that the threat posed by global warming to endangered species warrants us taking more dramatic and earlier action, than if our own survival was the only consideration. Such obligations can only be shared with other sapient species
Some would like to think of the intelligence of various species having a continuum type characteristic such that, if sperm whales prove to have reached our level, some other cetaceans might be not far behind. You might envision a grey area where these ‘in-betweeners’ can make rational decisions guiding their own future, even if their comprehension of it is limited, but I think it unlikely for the following reason…
A big brain has a high energy cost, and so is heavily selected against, unless the possible marginal extra use for it is of high value. With sperm whales we already know that point to have been reached because their culture is a more important predictor of reproductive success than their genes are. In humans we know that point was reached 600 thousand years ago, as that is the time from which our average brain size began a trend of strong growth – though the actual rate from that point is in dispute. Anyhow, no matter how we model it, we find that the transition from chimpanzee level to human was rather fast, such that there was only a very short window during which we might find anything intermediate.
So, I expect it is a rather all or nothing situation, just like many a religion has painted it.