Ok I agree with the overall point you are trying to make, but I wouldn't lump all those guys into the same basket. Lets go through them one-by-one (because thats how I like to do things):
First group - too old expect anymore development after 2010. (I note your comments about people being able to add facets to there game later on... but you cannot ignore the average trajectory of players - there is a reason players decline in their later years and are forced to retire - otherwise we'd have a bunch of 40-50 year olds still playing).
Johnson was 29 in 2010. Some of his best years were 2009-11. Far from being a guy that didn't have some sort of progress late in his career.
Jolly was 29 in 2010 - and was a great Ruck for us that year... his body shut down somewhere around 2012, which is as expected for a Ruckman... doesn't fit this argument at all.
Tarrant - returned to us as a 30 year old... had a very good first season back... can't really expect much more from him at that age.
L Brown - 2011 was his final year... don't see any way you can blame Bucks for anything here. And at that stage had showed that as a club (Malthouse?) we were definitely able to get the best out of him in his twilight years.
Maxwell - again doesnt fit the argument. IMO played some of his best footy in his final years (especially this year) - maybe not stats-wise, but definitely from a leadership perspective... wouldn't want anything more from him.
Didak - Can't blame coaching staff at all IMO for Didaks last couple of years. His body was shot, he had no lateral movement, and could not hold a tackle because of his shoulders - was basically a traffic cone on defence (and that was even when he came on fresh as a sub)... I was Did's biggest fan in his early years, and watched him closely at the end, and personally I think he was given too many opportunities - he was a liability.
Krak - really? I'm not sure what more you could've expected for a guy who spent 4 years away from the top level, and had ongoing family and health/body/conditioning issues. I think his 2010 Sandover and his 2011 for us were remarkable given what he had been through.
Now those guys are out of the way.
The next group:
Swan - was one of the absolute elite mids of the game from 09-13. (ages 25-28... actually a pretty late peaker)... and only last year did he drop off. Time will tell how much of that was injury/conditioning related. (I think his best is behind him, but should have a better year than last). Of the 2 guys of his ilk, Ablett has stayed at that level for at least 1 year longer... but Judd fell away a fair bit earlier.
Shaw - Like most here I am very disappointed in how things went with Heater - but who takes the blame for that? And we aren't talking about football ability with him. 2013 was primed for Heath to step up as a leader of this club... instead he showed the exact opposite, was bickering with his teammates for much of the year, and was a big part in us losing to Port in the final. Blame Bucks all you want for Heath not taking the step up, but IMO, those seeds had been sown many years beforehand.
Lumumba - Yep, here is one that fits your argument. (Probably the first on this list that I would agree to put down to footy ability/development). I've made it clear multiple times where I think we made a mistake with Heretier - he was at his best as a defender who made occasional booming runs up forward... we lost the balance somewhere, and once he lost his skills as a defender there was no going back to that role.
Dawes/Wellingham/Thomas - I have seen nothing from them at their new clubs to suggest that they had further potential for improvement that we hadn't tapped into... in fact at least 2 of them have declined.
Goldsack - I think he has made some developments since then - has added a bit of flexibility to his game, and has become known as one of the hardest tacklers in the competition.
Caff - again, I disagree and think we saw significant development from Caff. Missed 2 whole years of footy, and then became one of the best taggers in the game.
Toovey - agree he should of provided much more this year, particularly when the rest of our backline was so young... but again was a guy who most most of 2013 with injury.
Cloke - was disappointing last year... but 2010-2013 (23-26yo) were his best years... again doesnt quite fit your argument that we can't develop guys in those years. Still only 27, so I think we should see a lot of improvement moving forward from last year.
Nathan Brown - agree - this is a guy who showed a lot of development early, but then stalled and has not been at his best since. But again - has basically missed 2 whole seasons of footy since 2010.
Ben Reid - Probably peaked as a 20-22 year old. Was average in 2013, but we showed some creativity and used him effectively as a forward (again - that is some development). Again injuries.
Blair - agree would've loved to have seen more development from him... at the moment he's sort of stuck in no-mans land... provides enough to still be a regular in the 22 (but for how much longer?) but never took the step up.
Yeah, i'm not even going to fathom responding about Pendles. When someone is considered top 2-3 players in the game, it is incredibly harsh to say he hasn't developed. What it would be nice for him to add is the abillity to be a great captain... but as we often see that is a skill that very few players have, and is often more nature than nurture.
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So basically - of the guys who weren't too old, we have a bunch who have struggled with injury, and a few who had a poor 2014 but were very good prior to that. Only really leaves a few, IMO, who have disappointed in their development for a few years without having injuries to blame.
Thanks for taking the time to go through all this....I agree wholeheartedly and appreciate your thorough review.