First of all, being active on an otherwise not-so-active tag comes with some caveats.
- You won't get as much attention for your posts
- There will probably be less content to work on
I'm sure you are aware of these things better than me but that's what you'll get when browsing though those tags - it's something that most defiantly affect you "progress" on the site. One the other hand, being very active in an otherwise quiet niche will gain you the ability to become the "goto-guy" with regard to questions containing those tags.
With regard to your feature-request -
I would like to see the tag badges awarded on a sliding scale, based
on how many other users have the badge (or another, perhaps better,
measure of how popular the tag is).
It would complicate the calculations of the badges.
It makes no sense to have different criteria per badge per tag. That would make it unfair IMO.
You work within a very specific area - why should you get a badge for answering less questions and getting less upvotes?
- Because the tag is less active?
- Because there are not as many questions?
How does this make you more or less deserving of the badge? How would you suggest this calculation? According to the number of posts? What if there is a brand new tag with only 5 posts - would answering all of them get you a gold badge? I think you can see what I'm trying to get at.
It would not be feasible to implement different calculations - those calculations would have to change on a regular basis depending on the popularity of the tag - it would get really confusing.
Imagine that you were one upvote away from a gold tag badge in
FizzBuzz - but then some poor users come along and ask 5 additional questions in that tag - would you still be eligible for your gold tag badge? I guess not if the calculation is based on the popularity...