My understanding was you were just assigning 2 words which mean the same to two aspects one of which you support, one of which you don't. That by itself doesn't support or refute any argument though.
I looked up the words in dictionary. A niche is a specialisation. Now, what exactly does
flavor mean in your dictionary because I think we're using different dictionaries here. As far as I'm concerned it boils down to same in context of class and spec design.
Case in point: there are 3 energy-based classes and 5 energy-based specs. Feral druid, windwalker monk, and rogue (assassination, combat, subtlety). You can call these "energy flavors" or "energy specialisation" or "energy niches" it all means the same. But lord, wait, there's this thing called class specialisation. Yeah of course there is but
that's a different context!