The silent fix actually made the Mini 4 volume a lot lower than the Mini 2. I for one am glad to have loud speakers at the cost of a little vibration.
This vibration issue is something some people either cannot accept or are happy with. For me, personally, it is an unacceptable for a device which you hold. The whole chassis shakes even at 1-2 bars. What is the point of it shaking, if you are holding it for extended periods?
For me it is uncomfortable and it's like holding a speaker in my hands. It is also the principle of it that by keeping it I am telling Apple that I am happy with it, they will continue to make it this way until people complain. The fact that they have put in a smaller speaker in the iPad mini 4 and the vibration issue lessened significantly shows me that either enough people have complained about this issue or they've realised that this issue isn't up to the quality expected of Apple.
If you're happy with your iPad Air 2 vibrating, because you have a louder speaker, then you have product that you want. But don't expect everyone else to be happy with it, this is a known issue which a lot of other people have complained about.
I also disagree that the iPad mini 4 has lower volume than the mini 2. I have a mini 2 and tested the mini 4 today in the shop, and the sound volume is about the same.