You don't promise young players anything. They either want to work for it or they don't, that's all there is to it. Eddie came through with Auba and Laca at their prime and Martinelli being the new youth sensation, and he stayed and worked hard for his opportunities. Smith-Rowe didn't have an easy path at the time with guys like Özil ahead of him. Saka came through right when we went and broke the club transfer record on Pepé and Auba was the best striker in England playing off the left wing.
I think there are things you can do to motivate youth players without creating a highway for them into the first team though, and one of them is definitely to create an U23 atmosphere where it feels like they are part of something big. I think that was true of the U23 under Freddie for a while, but it has not been true since then. They need to be out there winning and playing inspiring football, not being so bland and disjointed that they have to borrow Reiss Nelson from the first team for half a season in an attempt to avoid relegation. Seeing the sheer difference between Chelsea and United's kids and ours this season has been disheartening. They played like actual teams and not just a cobbled together group of teenagers.
Another thing is that I think we need to sort out better loans. We ideally shouldn't be looking at League One and League Two for any 19/20-year old player. Give them to lower Championship sides or send them abroad. Think Goon is right too that we have to poach better from others. Möller was brought in partly to address this but we need much more transfer activity like that; players that are actually good and promising, and not guys that are released by Tottenham and get brought in to make up the numbers.
