In the context of humans, the concept of one-size-fits-all is a basic non starter.
[a fine point if I may: this piece talks about individuals and their uniqueness but it is obvious via observation that humans have many overlapping/shared behaviours, thought processes, beliefs, emotional responses etc. as well.]
Given all of the bits and bobs that go into creation of a human being, it stands to reason that the exact makeup of each individual is unique, even with twins et al. (Cloning might be an outlier here but a standalone topic.) The biological and chemical responses of an individual change continuously with time, environment, stimulus and regardless of the amount, the end result at each snapshot in time will be one of individual uniqueness.
Further, I think it is safe to say that for the most part, the phrase “the whole is greater than the sum of the parts" applies to human beings. Again, while we share many common macro responses, many common physical attributes and so on, when the proverbial rubber meets the road, 'the whole' has been arrived at as it were, the individual's experience/perception/emotional response, the entire shabang is unique to that person.
Building on this, let's switch to the perspective of the individual. Aside from some published accounts here and there in addition to the claims of many a charlatan, past, present and future, humans are not generally or overly psychic so it also stands to reason then that an individual's existence is subjective: only the individual can fully understand their personal/individual perceptions, experiences. This in turn means that the individual is the only one in the best position to make decisions for their own welfare. Outliers here could include mental health challenges including addiction(s) as well as young children. While the “in the best position to make decisions” may need to be adjusted in those instances for example, there is no change in ‘the state of uniqueness’ of the individual, only the input factors that combine to create/determine that uniqueness have changed.
Looking at an education system, ironically a system that should understand individuality since it is about 'education' after-all, we see the one-size-fits-all model in full display and full failure. Despite the efforts of true educators to overcome this by tailoring their programs accordingly and/or offering extra individual help for example, IEP's for students is another, the overall emphasis remains one-size-fits-all and educational curriculum is for the most part delivered as such.
Those who can't keep up with this silly model fall by the way side and are promoted right through the system out into the wider world…a world they are sorely ill equipped to deal with ‘cause one size fits all dontcha know. If this wasn’t enough there are those educations systems that demand standardized testing as does the Province of Ontario…*smh*, standardized testing for unique individuals. Seriously, y'all just look foolish really.
But it doesn't stop there in this instance. The manner in which the govt group assesses these tests does not take into account the varying scholastic abilities of the students taking said test thereby rendering the results skewed and worthless as a standardized metric. [as an aside, EQAO, the standardized testing program in Ontario makes for a good taxpayer funded collection of plumb positions, potential business contracts for govt group cronies and political optics just to name a few…a few things that have absolutely nothing to do with the educational success of the children/youth this system claims to exist for.]
Marketers and manufacturers: if one-size-fits-all, why would different businesses/companies even exist? There would only be a need for giant factories pumping out the various one-size-fits-all whatevers. Looking closer at marketers for example, why do marketing companies create campaigns targeted at demographic groups, often with entirely different messaging delivered to each group, if one-size-fits-all?
Setting aside the simple silliness of the entire concept, the one-size-fits-all model has some serious problems attached to it as well. For starters, it is dis-empowering. This is glaringly apparent when dealing with the govt group. When an edict from the throne is handed down that an individual in a particular region does not agree with, heck even when it's most of the individuals in that region that don't agree, there is little recourse available and the individual feels their voice makes no difference in the bigger crowd so they simply give in or stop vocalizing their dissatisfaction in the face of one-size-fits-all.
Another reason one-size-fits-all is loved by the govt group is the resulting group mentality making it easier to manufacture group consensus - cue the covid slogan for example: "We're in this together!". Armed with this generic call to action, the govt group will find it easier to rally folks to the banner, to the cause. For reasons such as a belief that ‘more means right’, not wanting to be left out, caught up in the momentum of the moment, afraid, just to name a few. Like the effects of magnetism, more and more individuals are attracted and a self re-enforcing loop is created regardless of validity. The govt group will then continue on their merry way of one-size-fits-all service delivery. On the surface of it, one-size-fits-all is the cheapest method for the govt group to deliver any of it's so called services while at the same time maintaining the appearance of ‘taking care of business' for the public. With the exception of tweaks here and there, quite possibly the result of delivering on political favours, acquiescing to loud special interest groups or even the odd idealist, the govt group's entire approach to everything is one of one-size-fits-all.
In conjunction with a few other required changes (ie. monetary system reform, genuine drive towards govt group reduction to zero) a true free market allows an individual to express themselves with each opening or not opening of their wallet, patronage of a business or not and so on. If they still feel their voice isn't heard, the individual could open their own business for example. The individual can shape and live their life as they see fit with minimal intrusions upon it by others.
You know what? I'd like to change my answer: there is a one size that fits all solution after-all and we call it freedom