Different people have different perspectives. If one has a bad back, I assure you it will probably feel like the end of the world by the end of those 12 hours. And when, in addition, you are only flying because you have to be at a business meeting in a few hours after landing to make a presentation, and the person behind you was kicking your seat the whole way because they didn't want you to recline (on an overnight flight!), or the poor person next to you had been traveling for days previous without washing, or there was a belligerent drunk arguing with everyone next to them (all of which I have experienced), then a reasonable person might start thinking that spending a little effort to get upgraded would be a wise idea. And there is also the greater protection of business class in the event of IRROPs (which has saved my agenda more than once) and showers in the arrival lounge.

Sure, 12 hours in economy might not kill you. I did it when I was younger. But if the point of the trip is to meet some business objective, and doing so will make it harder for you to achieve that objective, then you should perhaps reconsider whether that is the best choice.