The following assumes that you have not damaged your equipment. If you have recently dropped or damaged your camera / lenses, this could be your problem here …
This is the second Canon camera to do the same thing, the other is an EOS 40D. Both devices worked perfectly at the time of purchase and slowly failed. Does anyone else have any experience with a particular 60D or with a camera whose auto focus fails in this way?
Nope. I am currently sitting on a 20D, a 60D and a 5DmkII. All 3 work very well. (This is in addition to a Rebel movie from the early 2000s and from a 1V)
I can not use any of these lenses anymore because the focus is completely soft. He (Canon Tech) stated that when I use a third-party lens on my Canon camera, this may result in a camera malfunction or failure itself, requiring a repair from the camera. camera.
It's hogwash. Setting a third-party lens does not harm your camera (as long as the lens is designed for your camera). Third parties have been around for a long time – if there had been a systemic problem, it would have disappeared a long time ago. It's just that Canon does the old thing "we only support Canon things".
Also, is it time to buy a new Canon?
Not yet. See this question: How to diagnose the source of the problem of focus in a camera?
First of all, if you are not really taking images, so did you just accidentally set the diopter?
Second, if you're taking test shots, be sure to use a tripod and arrange the objects at varying distances so you can see where the focus is? For example, create a row of slightly off-center tins extending from the front to the back along a table. Focus on the center using autofocus. Take a picture. Revise it. Was the center centered or was it centered forward / backward?
Finally – do you monitor your shutter speeds while shooting? Are they at speeds that can be held by hand? If you are in automatic mode, you may just have an ISO problem that is too low for the environment.
Or should I sell all my gear and go to Nikon?
Until you diagnose exactly the cause of your focus problem, you should not expect to gain anything by changing the brand. It's like selling your Chevrolet to buy a new Ford because the car will not start (when you forgot to refuel!)
I mean, you could … but you can find yourself in the same situation because, well, you could be the root of your own problem.
Please download sample images to answer your question to further assist in the diagnosis. It's very difficult and too speculative without images. Please note the ISO values, Aperture, Shutter Speed, Lens, Focal Length for each test image. Also note that you used the autofocus and the selected focus point for the image.