I thought this might be good food for thought for my previous post about raids dropping the same level of gear. To simplify the searching, I've used plate chests as a sample. I might miss something here and there, but nothing should be far off.
Lowest blue plate (mail for before 40): 25
Lowest 'late game' gear (included due to being part of level 60 sets): 61
Lowest level 60: 65
Highest level 60: 92
The 'start to end' gap is 67
The 'noob to elite' gap is 31
Lowest level BC item: 81
Lowest level 70 BC item: 100
Highest level BC item: 159
The 'start to end' gap is 78
The 'noob to elite' gap is 59
Lowest LK item: 130
Lowest 80 LK item (excluding a random stat green): 200
Highest LK item (excluding hardmodes): 270
The 'start to end' gap is 140
The 'noob to elite' gap is 70
Or 77 if we want to include hardmodes.
Vanilla legendaries: 80, 80, and 90
BC legendaries: 156 (both warglaives) and 164
LK legendaries: 245 and 284
Note that the lowest vanilla legendaries are based on drops from the lowest tier raid and trash drops from early to midway through the second tier. The highest vanilla legendary came from the highest tier raid.
The BC legendaries were from last and second to last raid tiers.
The LK legendaries come from the second and last raids, respectively.
Clearly gear is going up, and quickly. But where is it taking place?
Level 60 vanilla gear and BC gear are off by 16, while level 70 BC and LK gear are off by 70. Note that I'm counting the 58/68 gear as 70 to keep things simple; the measure is essentially "if you were at 60 or 70 and not raiding, what gear would you be getting?" The gear resets aren't the cause, and in fact they don't even catch up to the highest level. The leveling process in BC didn't produce a wide range either, only 19, but gearing up caused a much larger gap. In contrast the leveling process in LK covers 70 item levels and then gearing up another 70, a similar gap as in BC, which is more than double that of vanilla.
TL;DR: The gear resets are not the main causes of stat inflation. BC gearing caused huge stat inflation. LK leveling caused even more stat inflation. LK gearing caused huge stat inflation, but no more than BC. This could mean a few things. First, the "twice as many tiers" hypothesis of LK inflation may be false. However if vanilla is the baseline, then BC had excessive gear gaps and LK may have attempted to fix this, but failed due to the "twice as many tiers" hypothesis. In other words, flip a coin, then go ask Ghostcrawler to pick heads or tails.