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.
Maybe $5,000 if you're talking about high-touch legal help or the trademarks are dicey.
If you hold a trademark you are obligated to defend it. If you don't others can void it.
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)
1: https://www.worldtrademarkreview.com/united-states-strong-ri...
Edit: Not copyright -> trademark
The point of open source is that you can modify it. But can you modify it and still call it the same name? If so, how much can you modify it?
> 2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
If I patch the Linux kernel and compile it, can I call it the Linux kernel?
If Streamlabs patches OBS, can they call it Streamlabs OBS?
There really isn’t a straightforward answer.