Flipside, how can allowing a companion to get pyometra be ethical? For some breeds the chances are extremely high and contribute to the low average life expectancies.
It's not like this is a clear-cut all or nothing for either proposition, there are issues that spay prevents (not so much neuter that I know of) and there are issues that it can cause. Risks are often breed specific and should be considered as part of the decision. Not spaying, especially in breeds such as the Great Dane, comes with a 62% chance of pyometra by age 10; avoiding that and the high liklihood of a miserable death is not insignificant. The rates of mammary cancer can also be significant; however, to see any reduction in the rate one needs to spay before the age of 3 according to the most recent papers I have read (not a vet, but I try to read papers that are relevant)
Edit: For hormone levels, there's the OSS (Ovary Sparing Spay) which retains the ovaries, but yeets the uterus; thus, retaining much of the hormonal balance without the risk of pyometra....in theory as I haven't read many papers on this. My understaning is that it does not reduce the risks of mammary cancers, so might not be the preferred option for those breeds where that is a major issue. I would appreciate info on OSS papers.