what makes you think that you want to change camera? these cameras are actually in the same grade, so there isn't a lot of UPGRADE if you get any of those.

both of the points you said are wrong.

true about the built-in AF motor, but that does not mean BETTER FOCUSING. it only means you can focus with AFD lenses. the term better focusing means more advanced focus system. for example, you say that the 1DX has better focusing than 5D mark iii,because of the number of AF points/ ability to track etc. same to the d90, it totally loses to the d5300 (which has the d7k excellent focusing system), not sure about the d3300 though.

and for the sensor, trust me, go any review sites and check the iso 800 performance of the d90 and compare it with the new 24mp nikon sensor. d90 is so old, and the sensor will NEVER beat newer sensors.

so based on what you say, the nikon engineers wasted their time and effort to implement newer autofocus system? and lets not focus on the built in motor or not, how many users are using D lenses? and how many D lenses are made for crop? and to bring things into comparison, a 700D will focus better than a 40D or something like that, even though they aren't the same class, but technology is moving fast, things will get better and better.

i can't really find any direct comparison for the d90 and the new 24mp sensor, since the d90 is so old. and you are saying that the d7k is better than the d90? the same sensor in the d7.1k is the one used in d3300 and d5300, so you are saying that the sensor in f7.1k is worse than d7k?

you must be using the camera in a wrong way. what aperture are you shooting with the 50mm? i have been using the 550D for 2 years, and totally no complaints in term of sharpness.

https://www.flickr.com/photos/83735357@N04/

all my photos are shot with the 550D, sharp or not, you decide.