The importance of depth is that while the "lesser" prospects may have a lower individual probability of success, having enough legitimate prospects raises the expected value of success for the whole organization.
Only a few uber-prospects have high probabilities of success and high ceilings. And that's luck as much as skill (did every team that passed over Trout have bad scouts?).
A decade ago, one thing that characterized the Phillies farm system was a total lack of depth, once you got past the top ten lists you were often looking at dregs, players with limited ceilings at best.
So yes, they need better drafting at the top, and better targeting of LA bonus babies (but frankly, outside of 3-4 players each year, I think the rest of the top 30 are equivalent to 2nd-3rd rd picks in terms of probability of success, because projecting 16 year olds is much harder than projecting 18 year olds).
The 2000s Phillies hit on a bunch of 1st and 2nd rd picks, but little else.
The 2010s have seen some hits later in the draft, and better talent out of LA, but too many misses at top,