The switch on the D7000 is always stiffer than any other of my Nikons but it never got too stiff. It is likely there is dried lubricant in the collar. WD-40 would not be good on any electronics but there are lubricants that would probable do fine. The problem with spays, it is hard to control the area covered and amount dispensed. An electronics parts store would have some chemicals that would be effective. One is a deoxidizing agent named D-Oxit by Caig Labs in the US and Cramolin in Europe. It is available in spray and in liquid form. The liquid form when applied with a needle dispenser right at the shaft and collar would surely do it but the knob is in the way for access. The proper way would be to remove the back case of the camera and apply it to the shaft/collar directly.
If you are handy with tools it is not that hard. When I got my first digital camera, the D90 when it came out, I took it apart and built in a wireless flash controller I designed and it is still in perfect operational condition 390,000 frames later. My D7000 is my loaner, the camera loaned to people who beg to use my cameras for some project and no one touches my D800. D850 or Z6
It helps to have the service manual for disassembly if you are not familiar with how Nikon's are put together.
I have the Service Manual but I don't see a way to upload non-photo files.
Here is a link to a PDF
https://elektrotanya.com/nikon_d7000_sm.pdf/download.html#dl
If you have opened cameras before you will notice Nikons are built really well compared to some other brands. There are also access to parts which some brands intentionally limit access because they do not want cameras repaired and expect users to buy new cameras...hint...Sony
The link about takes you to a page to down load. It will start a timer of about 10-15 seconds before the actual file is available to download, to prevent robots from downloading thousands of files, the actual button to download becomes available by scrolling down the screen. It is sort of a collective where members, mostly electronic technicians and engineers share their service manuals which are hard to find. I have about 40,000 service manuals for electronic test instruments and professional audio equipment. If you have trouble downloading send me a PM and I can send it directly as an attachment, 2.3mb pdf file.
As a general rule, anything that is labeled "contact cleaner" is usually the worst thing for contacts. The most common cause of "cleaning" of contacts, relays, potentiometers and jacks is not dirt but oxides forming on the metal. Most chemicals labeled as contact cleaners enable more oxide to build up, and oxide of a metal is an insulator. So the switch or connector might be "clean" but it will not pass electrical current. So another class of chemicals were developed that attacks the oxygen that bound with the metal surface for a few molecules thickness. Once the oxide builds up deeper, low voltages found in electronics, can't penetrate the oxide layer. Power switches or contactors with 120-230 volts however can jump through the oxide layer so electricians use it but it is horrible for electronics. The same oxide that renders volume controls and signal switches intermittent of scratchy sounding can also build up enough to freeze the shaft of rotary switches or volume controls. If you ever had a stereo where the treble control or volume control was sluggish and made scratchy static sound through the speakers, that is Oxide and a small squirt of DeOxit will remove the oxide and leave a protector film that returns normal action for at least 6-12 months or more. So getting some DeOxit into the shaft of the power switch will free it up and protect the electrical contact for a long time. Oxides build up on almost any metal exposed to atmospheric oxygen. Aluminium is grey because it oxidizes so quickly but just smelted aluminium is naturally bright shiny mirror like and to protect the metal surface they actually bath it while still hot, in oxygen to corrode the surface to grey, so other reactive agents do not corrode it. Oxide is very important in electronics, all semiconductors like transistors and ICs depend on it to function.
Good luck....The D7000 is a fine camera and I still use mine occasionally.