I support the OP.
While it is true that longer snorkels are all but impossible to find, I believe that for some people in certain situations they would be quite useful. And I am one of them.
On many occasions, I wished I had a longer snorkel. When I float around reefs in rough seas, I often have to purge every time I exhale and this is both annoying and exhausting. In such conditions if my snorkel were just one inch longer, it would make my snorkeling both safer and more enjoyable.
Fairly confident you will not find snorkels any longer than they currently are. When you breath with a snorkel, you are extending your airway by roughly 12 inches. After you exhale, as you inhale, you are rebreathing your expired air for the length of the snorkel mouth and esophigus until you breath more air volume than is this airway. If you still have your OW book, you should find this issue documented.
Sorry
The volume of dead air a person can handle is individual. Among other factors it depends on the volume of the person's lungs and the breathing rhythm. Logically, the more air one in/exhales with each breath, the smaller the proportion of dead air in the lung.
I have encountered people who have difficulties breathing with a standard length snorkel. Often they are shallow breathers and when they start suffering the symptoms of hypercapnia, they instinctively increase their breathing rate exacerbating the problem and end up tearing the snorkel and the mask of their faces. (Interestingly, once the mask is off people take a deep breath.)
People with a sufficient lung volume (and possibly other physiological qualities I know nothing about) obviously can handle longer snorkels, and so do people with smaller lungs if they breath consciously.
To me the disadvantage of a longer snorkel would be rather the difficulty to purge after a (free) dive. I regularly purge twice: surface, blow, inhale slowly and carefully, blow again. If my snorkels were another 6" longer, I'd probably have to take it out of my mouth and let gravity do the work.
In "normal" snorkeling conditions the standard snorkel is ideal. But a longer snorkel would come handy on a windy day. I am actually considering of duct-taping a section of a snorkel on top and try it for both snorkel types: with and without valves.
