Totally agree, playing with seasoned pros in competitive games must have been of massive benefit to young guys coming through, and many ex pros have come out and said as much.
I think sometimes the Scottish mentality can be a bit of a burden, where perhaps boys of that age can get a wee bit distracted by off the field stuff and the peer pressure of their mates. Plus, having a grounded, stable family background with supportive parents and family can make a difference too. I know of one young guy who walked away from a deal a couple of years ago with Celtic because his junkie dad thought the money was not good enough, and is now playing Juniors. Must be many sliding doors moments like that in the game.
Up to age of 18, most Scottish players can compete with the best of the rest in Europe, but players from other countries tend to push on that wee bit more between 18 and 21, albeit there exceptions like Gilmour, Patterson and Lowry, who all look as if they are going to have successful careers. But there must be a reason why so many don’t push on. You would think the SFA and the clubs would ask both themselves and the players why this is.
Another possible related issue is that many countries like England, France, Holland, Germany etc have benefitted from immigration, wherein many 2nd and 3rd generation immigrants are now playing for their country. Scotland has not yet had much benefit in that regards, and is not until the boys mature into fully grown adults that you start to really notice the physical differences between nationalities in the modern game, and that can push Scottish teams to look outside Scotland to get players that can compete both physically and athletically. The further down the game you go in Scotland, you start to see massive gaps in both physique and fitness.
For me, a good idea might also be to send some young guys on short loans abroad, away from their mates, and force them to grow up quickly.