https://en.m.wikipedia.org/wiki/Curb_cut_effect

This might be it, although it might be specific to disability.

• ex. Closed captions, cuts in sidewalks

I believe it's called Universal Design. You can hear about that and curb cuts specifically in eps 308 of the 99% invisible podcast.

One of my coworkers was talking about how his wife (a truly hateful woman) was complaining about having been to a bathroom at a particular airport, and how they had changed them for trans people (she presumed). In particular, they had made all the stalls have floor-to-ceiling doors for privacy. I responded "wait... she's mad because they made the bathrooms better for everyone, because they did it for trans people? That is an objectively better bathroom situation. I can't imagine being upset by that."

People can still manage to be upset, but if they did do that for trans people (and I'm honestly not convinced), that's fantastic, and is a perfect example of what you're talking about, I think.

In addition to a word for it, which I see you've gotten answers to, this comic is popular to illustrate the effect: https://i.imgur.com/LF2Avja.jpeg

"A rising tide lifts all ships"

Collateral remedy sounds nice :-)

Socialism

"The curb effect" for the little Ramos people made on curbs for wheel chairs, that turned out to be useful for delivery people, baby carriages, bicyckes, etc

In the disability advocacy community, we like to say that accessibility is for everyone!

I tend to think of it as positive synergy but you’re right, it does need a more defined/known term.

Affordance? I've heard of it when talking about tools or utensils that are universally usable.