Out of those three projects, two are notoriously under-resourced, and one is notorious for constantly ramming through new features at a pace the other two projects can't or won't keep up with.

Why wouldn't the overworked/underresourced Safari and Firefox people want an excuse to have less work to do?

This appeal to authority doesn't hold water for me because the important question is not 'do people with specific priorities think this is a good idea' but instead 'will this idea negatively impact the web platform and its billions of users'. Out of those billions of users it's quite possible a sizable number of them rely on XSLT, and in my reading around this issue I haven't seen concrete data supporting that nobody uses XSLT. If nobody really used it there wouldn't be a need for that polyfill.

Fundamentally the question that should be asked here is: Billions of people use the web every day, which means they're relying on technologies like HTML, CSS, XML, XSLT, etc. Are we okay with breaking something that 0.1% of users rely on? If we are, okay, but who's going to tell that 0.1% of a billion people that they don't matter?

The argument I've seen made is that Google doesn't have the resources (somehow) to maintain XSLT support. One of the googlers argued that new emerging web APIs are more popular, and thus more deserving of resources. So what we've created is a zero-sum game where any new feature added to the platform requires the removal of an existing feature. Where does that game end? Will we eventually remove ARIA and/or screen reader support because it's not used by enough people?

I think all three browser vendors have a duty to their users to support them to the best of their ability, and Google has the financial and human resources to support users of XSLT and is choosing not to.