Who says they aren't ready? They probably thought Cumberland wasn't ready or of 'middling ability' and he proved them wrong. You won't know unless you give players a shot. You're effectively advocating for known mediocre quantities over unknown quantities with the possibility of improvement. That's a sure fire way to the bottom of the ladder.
I actually think over the last 4 or 5 years, we've generally done well in playing young players. However our team selection at various points over the last year or two has been baffling though. For example, Soldo as sub, flip flopping Balta, Ryan debuting in a tsunami, continual selection of Castagna until recently, Baker being stuck at the forward pocket, Rioli Jnr being dropped, playing a crippled Lambert, playing a cooked Edwards.