The "make sure these players get at least the first 15-20 mins of the game to stake their claim to stay out there longer" doesn't really work unless you are in a competition with rolling substitutions.
The u18s coach at our club (State League Div 2 - WA) is new to the role this year. (He is also the club's technical director) Although he has his C licence and has vast football experience, his coaching experience has been with skill acquisition age groups. He correctly considers that this period (9-13 years old) is the most important developmental stage but so much so that he completely discounts older players (ie 16-17 year olds) who he considers have "missed the boat" at that stage. In his words "it is too late for them".
As a result, he rewards talent over hard work, picks players week in week out who barely make one training session a week over players who are training 2-4 times a week without fail. There is also a core group in the squad who regularly just wander off after training while other players pick up cones, collect balls, put goals away etc. Unsurprisingly, it is the players who train regularly and work hard who are the ones pulling their weight in the packing up.
In my opinion, the message that this sends is that hard work doesn't matter. The talented players know they will get away with slacking off; that training is, in effect optional, and as a result, they train less, develop less and their performance and the team's performance is negatively impacted. The hard workers who are training 2-4 nights a week to get 15-25 mins off the bench become demotivated and miss out on the important development that only comes from playing games.
In conversations I have had with the coach, another contributing factor is a positive discrimination towards players who have come from a disadvantaged background. Their attitude and workload shortcomings are overlooked, whereas the hard work of middle class white kids is taken as granted.
Yes, it is State league but it is also the only u18s team in a community club which fields eight mens teams from State League down to metro (ie social) leagues. Not every player who comes through the 18s will be or even has the potential to be a State League/NPL or higher player. Some will just enjoy playing amateur or social football for many years because they love football and enjoy being part of a club. In my experience, these players will generally be the ones who do the heavy lifting at clubs on committees, coaching juniors etc.
A policy whereby training attendance and performance is the primary selection criteria send a number of powerful messages. Most importantly, it manages players expectations and is fair. Training is no longer optional and yes, there will be occasions when good players miss games or are on the bench because they have been unable to train for entirely legitimate reasons (this age group is obviously heavily impacted by study commitments) but it will also greatly reduce the number of training sessions missed for no/poor excuses once players realise that they will not start that week.