Camera Straps do wear out after extended use, check them regularly for signs of wear and tear. A few dollars spent on new straps can save you hundreds in repair bills.
The first thing that wears out is the "grippy" material that Canon uses on the underside of the strap. I had a few old Lowepro straps that came with the holster style cases I use. The Lowepro "grippy" material is far superior to what Canon uses on the original equipment straps for the Mark II straps, so I cut the Lowepro "grippy" stuff off of the straps I wasn't using and threw them into the bottom of my camera bag. I had one sewn on by a seamstress in the Philippines and the other by a nice dress maker in Phnom Penh Cambodia.
After a few years of constant use, my rather heavy Mark II's with the 70-200 and the 16-35 attached took their toll on the nylon straps at the point where they were attached to the camera body. Once my straps start to show wear, I switch them from one body to the other. This places the wear point on a different spot on the strap and allows you to use it longer before needing replacement.
Below, see a shot of the straps when I decided it was time to replace them. The strap under the word "Canon" is the one that holds my camera with the 16-35, and under the word "digital" is the strap attached to my body that holds the 70-200. You can see the difference in wear caused by the heavier 70-200 lens.
The strap holding the body with the 70-200 was hanging on by about 20 percent of the material and I was getting worried that it would fail, so I ordered two new Canon L4 straps from Amazon. The L4 strap is black and white (gray actually), it's narrower at the shoulder and lighter weight than the original straps that came with the Mark II. I'll be trying out the new straps while on an upcoming assignment in Burma, Thailand and Cambodia over the next 8 weeks. If I notice any advantages or disadvantages of the Canon L4 versus the original ones, I'll post it here when I get back.
Below, you can see the difference in width (slightly narrower) of the Canon L4 strap versus the original equipment that shipped with the Mark II.
Personally, I'm not a fan of any type of strap with a "quick release" or some sort of shock absorbing elastic component. I like the "solid", positive feel of a non elastic strap and I don't want to worry about a "quick release strap" accidentally releasing. There are plenty of aftermarket straps out there to choose from, but the Canon L4 is my recommendation.
I hope that this information has been helpful to you. If you decide to purchase new straps from Amazon, I'll get a small commission if you click through to this Amazon Order Page ...Thank you!