I think the situation on the FOSS side is compounded by the fact that gdb, while it's unquestionably not in any way "state of the art", is also not totally useless, so there's less of a reason to create something that's truly excellent. Moreover, there are whole generations of programmers now who think that tooth extraction compares favorably to using a debugger.

If there wasn't gdb/lldb at all, I'm pretty sure Linux/BSD would have a debugger that's far better than any closed source counterpart.