Very few non-corporate FOSS projects would have the funds to file a trademark, especially an international one considering one can be infringed from anywhere around the world. And time spent on filing would be time not advancing the actual project, unless a lawyer is hired, which makes it even more expensive.
Even fewer would have the resources to actually pursue legal action if the trademark is infringed upon.
This is probably not a realistic suggestion for all but a few hundred projects.
Trademarks don't need to be registered and many trademarks simply exist through active use.
However, your second paragraph is the key one. While trademarks don't need to be registered (and, in any case, trademark registrations are mostly on a country by country basis) it's pretty much on you to spend the money to defend your trademark if you want to do so. No one's going to do it for you.
It costs less than $2,500 to file trademarks in the US, EU and UK which is plenty of coverage. OBS has received hundreds of thousands in funding, from YouTube, Twitch and Facebook as well as smaller patrons.
Maybe $5,000 if you're talking about high-touch legal help or the trademarks are dicey.
$2,500 would be about the average monthly gross income of a person earning $15/hr. And in terms of expendable income, that's an order of magnitude away of what people can shell out.
This is enough of a problem that anti-SLAPP laws have been passed in several jurisdictions. Those laws only go so far, though (and you still need to mount a defence of some description)
OBS is the #10 most popular project on Open Collective[1] (plus other donations), so yeah, it is one of the top-earning FOSS projects that can afford it.
OBS is probably well-known enough to have decent trademark rights just based on marketplace awareness [1]. You don't technically need a registered trademark if everyone recognizes your name (though it does help)
For example, when someone would bundle OBS with a virus and create a fake website with OBS logo and everything, one could sue them for c̶o̶p̶y̶r̶i̶g̶h̶t̶ trademark infringement. They would still be allowed to do that with FOSS software, but would need to create their own reputation.
FOSS doesn't really have permissive copyright though. You can't claim to be the author of a FOSS work that is not your own, even if it uses a permissive license. The license and the copyright are somewhat orthogonal.
