Not really true. The 17-year-olds that are decent prospects generally pitch a decent number of innings in the DSL. Yes they pitch more as 18-year-olds, but that is true anywhere. And a bunch of the players who are 18 are in their first season anyway.
No matter what you say, there ARE limits on practical playing time and coaching. There will be roster limits in a few years. We have 2 teams down there and probably can have 70-80 players in the complex. More than that and the better prospects don't get playing time. Many of the older 19 and 20 year old pitchers you see down there are signed inexpensively when they are older. This is an important new strategy of maximizing signing dollars and it is a good strategy. But it also does not speak to the problem of identifying good pitchers when they are 15 or 16.
Having lots of options is good. I am fine with more $500K signings than $2 million signings. I am fine tilting the bonuses 80-20 towards hitting which has less risk. I am not fine of your strategy of just saying the Phillies will win because they should sign hundreds of $10K guys. That just does not work in practice and there really is not much sign we are great at developing anyway. Our strategy should be spending to the max. Identifying tools and projecting them. And hoping we sign players who have high baseball IQs or intangibles that will help them if they do fill out (like Ranger). We signed him because he was a smart, athletic baseball player. We had no idea he'd be throwing 93-95 since he really was not the highly projectable player. We got a little lucky and the player probably worked really hard there and was coached well. There just are not many players like him that can be identified easily when they are 16. He at least was smart enough where his performance stood out even when he was throwing 85-88.