I suggest that generally, the portions of the flight for which it is prudent to have your shoulder harness snugged right up (takeoff, landing, and impending crash), would generally not be occasions where seleting fuel tanks is vital. I conceed that it could be prudent to switch off the fuel just before a crash, and you're already snugged up, and cannot. It's not perfect.
The 172, like most legacy GA aircraft, was designed long before shoulder harnesses were considered a worthwhile addition. Therefore the mutual exlusiveness of the shoulder harness, and fuel selector, probably just was not considered.
Wait 'till you fly an earlier 172/182, or 180/185 with manual flaps, you'll be reaching down there regularly then!
Shoulder harnesses can be left a little free, as long as you snug them at the right times, don't worry too much about this....
Yes, I have also heard that the John Denver's accident was control location related, but remind yourself that he was flying an amature built aircraft, and therefore that aircraft had not demonstrated design compliance with requirements for cockpit features. The 172 certainly has. Certified aircraft have met all the applicable design requirements, including those "little things" like fuel selector position. Amature airraft, though potentially very good aircraft, may not meet those requirements. The pilot is therefore more "on their own" to figure things out.