I think a great deal of our perspective on the rate of content balance change is relative to the rate of content release...
If you had a tier that you were in for six months before the next tier came out and it started getting a stacking nerf of 5% at three months, and it increased every month, you would be at about 15% or 20% at the end of this tier when the next tier is released. Relative to when the next content is made available, this may seem like an acceptable rate of adjusting the content balance.
Now if you were stuck in the same tier for nine months though and had that same schedule of re-balancing, suddenly you are ending at a 30% or 35%. Because the duration of this tier of content is longer as a whole, the speed at which the re-balancing is applied likely seems too fast.
How quickly the next content is released is probably one of the largest factors in what the majority of the player base would consider an appropriate rate of adjusting the difficulty. Players don't want to feel like they finished the content too fast and are just waiting for the next content. Likewise, players don't like to be left out feeling like didn't finish the content and are already pushed into the next. And of course you'll never be able to please everyone.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.