What percentage of your users use Safari? If it isn't impactful to your business, you could ask them to use Firefox or Chrome.

I realize we need more browser diversity and not less, but at the same time we should encourage all participants to keep up so that the burden doesn't fall on smaller players.

Pro tip: if you ask a user to install a different browser, they're just as likely to use a different site instead.

Depends on the market; I don't think it's that common in enterprise, for instance, to have Windows 10-using orgs with virtually no use of the default (Edge) browser, but large numbers of users with both IE and Chrome because of (legacy and modern) app demands.

Depends on the user. My GF for some reason uses FF, Edge and Chrome at the same time, most of the time (she's not in/into tech).

So if someone asked her, she would just switched the windows.

That sounds like a fairly rare workflow. I've heard of people using different browsers for different things but I don't think it's common.

I guess some people don't care about the browsers very much, and will just use whatever is installed. If there are multiple browsers installed, they may learn to use different browsers as a simple way to have different easily distinguishable browsing contexts (different icon/app name) - instead of having multiple windows of the same browser that looks pretty much indistinguishable.

  they're just as likely to use a different site instead.

just as likely, or more likely?

Depends on the site. In some cases, less likely. E.g. at this point in time, Facebook or YouTube or even Slack could make a browser change demand with little to no consequences.

YouTube would get the antitrust hammer thrown at it. Probably same with Facebook.