Another downside of pronoun declarations is that they bring extra attention to gender/sex in contexts where gender/sex isn't supposed to matter. Bias tends to be triggered in proportion to attention; one of the most effective ways to overcome bias is to focus attention on other things. So this is a (probably inadvertent) bias-enhancement technique.

But there's an upside that you don't make explicit enough: widespread pronoun declarations do give cover for people who might be identified differently than they want, so that they can put the information out there without automatically having people draw the inference "something's weird about me because you will get my gender wrong if I don't tell you".