Best quote I heard from Wenger on the topic was:
"You pay for youth players with points."
Absolutely correct, one moment of indecision can cost the game, fortunately (for the better teams) they have players usually capable of recovering from little errors before the damage is done.
Regards to Walsh, I was fortunate enough to have a long chat with Martinez during his second season here. He said the problem was they didn't know where to play him. He was quite pessimistic about his chances tbh. (Which myself and those with me found surprising as he was absolutely the eye catcher around that time.)
Yes thats a really god insight on both points. You look at Davies and he has cost Everton points. The PL is really ruthless now and you get punished for mistakes. If you look at Bournemouth for example, they will often ply lovely stuff but throw it away at key moments. We were quite similar under Martinez.
What you probably saw with Arsenal was they had a fantastic backline and goalkeeper to begin introducing such players. So as you say they had the insurance in place. Once that went, Wenger really struggled to emulate his earlier days. With United, while the young players were introduced, I always remember Paul Scholes saying (when someone posed the Alan Hansen, win nothing with kids tagline to him) that actually was right. They won the league because they still had good senior pro's, Schmiechel, Cantona, Bruce, Pallister, Irwin, Keane etc. It was those players who really won them the league, and the first 2 mentioned were top class in that season.
It also comes to pass, that if you have a cycle where managers are increasingly on shorter and shorter time frames, they will look at a young player as an investment that even if it comes off, comes off for someone else. If you had a pot of money for example, would you invest it in a scheme that paid off handsomely in 5 years, if you knew you could only access the returns for the next 2? Or would you put it in one that gave you what you needed for 2 years?
On Walsh I can imagine Martinez saying that. He needed to keep growing and had he have waited until this season when he was maybe 22, perhaps he'd have got a look in? However it's a big ask for a player and I never begrudge any of them that say they want to go out and play regularly. For every Osman that breaks through at 22/23 there are 10 Garbutts who have just wasted their careers.