Well, this can be summarized as “Fits will vary between riders”. The idea that pro fits aren’t necessarily ideal for average riders may not be true across the board, but again… that matches the reality that there are no absolutes in fitting.
When we dive into what we see in common pro fits, and then try to apply that to non-pro riders, LOTS of variables come into play. General fitness, flexibility, age, performance goals, comfort goals, etc. and more are all factors that come into play.
Generally speaking, Pro’s are pro’s for a reason. They are different (better?) than 95-99% of the related common population. That comes into play with fits in their abilities to live and deal with certain fit choices. The small frame trend you mention is largely about getting lower stack height frame, in order to get very low handlebars. They tend to couple that small frame with “abnormally” long stems as well, to get the right saddle to bar reach, while getting a very large saddle to bar drop. All that is most often aimed at aerodynamics for peak performance.
Take that same setup with an identical sized rider in his 40’s, who likely trains 10 hours or less and lacks the peak physical abilities and flexibility of those 20 something pros… and you often get a recipe for pain and discomfort in short order.
Typical riders aren’t looking to nail the marginal gains sought by pros. Getting a fit that is more aimed at “comfort” to state it simply, is a better deal for the greater public.
All fine and good for any rider to follow that pro trend if it works for them. A good fitter will look at the rider in front of them, couple that with their goals and end up with the fit that is “right for them”. If that happens to mimic a pro setup, then great. Fitting to a template without those considerations is a mistake, IMO.
There is not some hidden agenda by fitters to keep people from pro fits for some nefarious reason. But it doesn’t make sense to slap someone on a pro fit if/when we know or strongly suspect that they will return shortly with complaints.
Does that make sense?