An opinion.
You are never going to get a rule that works in all cases, there are too many variables, the fishes metabolic rate, their mass vs length ratio, their behaviors, the type of food they eat... Additionally, you then have to consider, the water change schedule, the filtration, the water chemistry, dimensions of the tank etc. This to me is really where the interest in keeping fish comes from.
The rule is appropriate for a beginner who is starting with little information, not for many of the people on this forum. The rule is simply a baseline and a better one than the old ones we used to have (I have fish books that recommend 2" of fish per gallon). Often a suboptimal, good enough, solution is better than no guidance. Most people start raising fish without any guidance other than from the LFS (no data to back up this opinion).
Ideally one could provide beginner tables starting from two different starting points of combinations of fish and tanks that work together. The tables would start with "I have a tank with these dimensions, what can you put in it", or "I would like a tank with these fish what size do I need" The problem with that is that it would provide even more opportunity for more debate. The other problem is with convincing people that they shouldn't do what they want to do, there are always contrary examples.
The LFS's can be a problem too, recommending inappropriate fish, sometimes directly countering my concerns.
The AqAdviser is definitely not perfect but it is better than nothing. I think it is a good starting point for a lot of people.