The idea that any of these guys are being held back doesn't fit with what I'm seeing on the pitch. Martinelli and ESR can't shine if they're injured or in poor form, and there is constant criticism of the coach when Saka plays a lot. Nketiah found some form for a few months but has struggled physically and technically in the past - he's finally bulked up a bit and looks capable of imposing himself on games, but that came with time.
If we sign a better left-winger than Martinelli and a better lef-8 than ESR, that's better for Arsenal. The project isn't about the players as individuals. Relying on younger guys who might not be able to hack it isn't brave, it's foolish.