I'm sure there are other potential vulns for higher firms. It's just that 3.0.0 and below is much more interesting to look into at this point in time.
Plus it seems like the groundwork is being laid out for future homebrew. What with libnx, reswitched's library, and a homebrew launcher in the works. Better to have the foundation for homebrew done before poking holes in later firms imho. That being said, if you ever do want homebrew, just stick to whatever firm you're on and wait it out (as I'm sure you've read multiple times).
TL : DR
There are likely more vulns, <=3.0.0 is more interesting for development. Maybe higher firms will be investigated when homebrew foundation is laid out.