Your analysis and prognosis is a reasonable one. However clubs buying and selling strategies don't exist in a vacuum nor do they have limitless amount of capital to spend. If we had both of those things, of course all you would do is buy the best 18 year olds and train them up.
The reality is, there are varying costs for players. We like every other clubs also have to work within a budget. The smart way to operate is to try and identify where value is. Most clubs are looking to sign younger players and it has inflated the market substantially, meaning there is some value to be had with players who are a bit older. I think Schneiderlin falls into that category. I also think Williams does, and so too does Fonte at 5 million pounds.
When you consider what Tompkins went for or Stones it's hard not to say Williams is an astute buy. At 27 all of the studies you produced showed that players still have 3-4 years of performance above that of a 23 year old remaining. At the prices we are being quoted it makes sense to do business. In all honesty we couldn't afford a lad with Schneiderlin, Williams, Bolasie or Gueyes ability who was 23 or under. So we either sign inferior players or players we can buy now and hope to continue to make money from our academy/other avenues.
This is precisely why the ability to identify and develop talent is so unbelievably valuable, yet they are things that remain undervalued in so many professional organizations. If you pull someone like Gana from out of nowhere each summer, you're miles ahead of the competition.
There is no argument against what you've said to the price:age ratio problem. But there's still a philosophical difference.
You have £25 million to spend and you get a 27-year old midfielder. Hopefully he produces like a £30 million player for 4-5 years and then you have to replace him.
Sometimes that's the 100% smartest way for a club to spend £25 million, and that may even be the case with Schneiderlin.
But we also know that there are players like Gana and John Stones who are moving every transfer window that are going to give you a greater return on your investment.
So if I have £25 million to spend, do I really want to spend it on a 27-year old (or £8 million to spend on Ashley Williams, etc)? Sometimes. But other times I'm going to hope my scouting and development teams are able to produce better returns.