While I sorta agree....I think you are getting better because you are gaining confidence in your shooting. The D7100 probably helped in that area too. But I believe you have improved way more than just a new body would have helped. You have come up with some amazing shots. The camera only saved them for you.
Yes we all get better the more we shoot. The D3100 is a fantastic camera dont get me wrong, I took some good photos with that camera. But there is more than just the better IQ going up to the D7100, it has a lot more features, a lot of them I will never use, but things like bracketing and the improved focusing system also make it a more desirable camera.
It all depends where you want to go with your photography I guess.
When I bought my D3100, I wanted it just to take photos for a website I was putting together, I thought it would be more than I would ever need. It served that purpose very well. But in the process I got bitten by the photography bug again ( I was years ago, in the film days as well ) So I seem to be constantly on the upgrade path now.
I have forced myself to stop buying more lenses for now and just use what I have and try and get the best out of it before I buy anything else.
But Oh how I want to buy that 11-16 & the 105