Strength 10 in GURPS was derived from what is considered "Fit" for military service. Given the increasing sedentary professions found in the modern world what we consider as "fit" is not as easy to come by these days.
Archeological Evidence and Modern Biometrics. I've pointed out that the Roman Soldier was around 5'3" on average (late roman), at a BMI of 18 (because of their diet and work regimen; BMI 18 is nearly skinny) and roughly 133lbs. By military biometric standards, these guys operate at what GURPS considers ST10. What makes a professional and well conditioned soldier, markedly different from their peers isn't much actually. They both do a lot of back breaking labor (professional soldiers DIG!).
A soldiers workload depends on the Historical Era and Circumstances. I can go on and on about that (which I did thats why I rewrote deleted the previous text) so I'll just go to the bottom line: Health is the measure of these Conditions. I would give Augustines and Marius' Reign and changes to the Roman army as points where there was an average HT12! In times of Roman Civilwar and Hanibal's wins where professional soldier manpower were drastically low the average was 10.
Feudal Levies fall under this same poor category, particularly since the warrior elite didn't pay much care for them, on average (while I've heard of some rare exceptions). Levies surely fall in HT10 caregory... until you have the more prosperous and liberal (as in a lot of individual rights) city states and freemen making up the levies (more of volunteers) where HT can be 11-12.
A man who came from the warrior elite of the past, who stands roughly 5'7", who has a BMI* of 25 (muscled), and weighs in around 160lbs. In GURPS this is ST11. I do want to note that the JUMP from ST11 to 12 is about 160lbs to 190lbs. A character who is 5'7" and gained 30lbs of muscle would be very thickly built if you use the imagination tools below** he looks like someone build like a wall.
Bottomline. Soldier Templates operate at the ST10 to ST11. ST11 being Warrior Elites or Modern Soldiers. ST12 and up are reserved for characters who are extremely tall and extremely muscled. Since the advantage height and Strength is a rarer deviating phenomenon in templates that reflect what was common and average, HT becomes a more effective means of group qualities of different forces.
Conclusively, ST10 and 11 will be group my templates for Low Tech between Elites and non-Elites/Commoner templates. Note that time progresses to more contemporary eras, most templates begin to fit the ST11 average.
*BMI doesn't distinguish from the quality of the extra weight.
**Imagining your character's muscle build. Here are where benchmarks are useful, they give context to numbers. Take the BMI of your preferred build. take for instance Strong Man champion ex. Mariusz Pudzianowski has a muscle BMI of 41.3. Bas Rutten who is the same height has a BMI of 27. your imagination can fill out the details.
Strength 9 is not the average. Anyone here who's struggled with diet and exercise will note how much everyone else is struggling. In our modern surroundings, a ST9 seems way more common than a ST10. This can be compounded by the factors of social economic class and current economic trends, more education and more technical jobs that leave all the heavy lifting to automation and laborers.
I guess Strength 9 is appropriate as the average strength desk jockeys. Although despite our surroundings, ST10 is the average strength if you look at the population of many emerging economies and the kind of work they do. To the majority of the world, work behind the desk even one that answers to irate foreigners (the important role of BPO in emerging markets) is still a cushy job by the standards of most of the people in the world.
As for future trends, the picture can be only answered by an honest "I don't know".