I'm not sure of the point you're trying to make...so don't take this the wrong way. Not looking to flame or troll.
I'm not an old dude who is falling apart, but I do have some physical failings, and for a very long time I was also (like the poster) in excruciating physical pain due to a back injury. If you don't have one, you don't know what it's like to wake up every morning knowing this is as good as it will be all day.
I'd like to say I know what helped relieve my pain, over time, because I'm actually pretty good now. Could be any number of things. I happen to think it was losing 30 pounds and getting to a BMI of 25 (I'm 6'0, so it's not like I was terribly obese, just slightly overweight). My back pain came from playing sports. I had to give up golf, among other sports. It could also have been relieved by a fitness trainer helping me focus on my core, someone I used for a year.
Point is, you can be in really, really bad shape and still want to have sex. After all, at my worst (over 10 years ago) massages for pain relief led me to mongering, and mongering was always a highlight (wife didn't understand my issues and wasn't particularly caring). Of course, it also almost ruined my marriage...so there's that...
I get where both sides are coming from on this point. But I can sympathize with the poster. It's hard not to if you've been there.