I agree with your general assessment of our farm system (it's pretty thin and lacking in big upside other than Maniger and Lewis)
You say, however, that the Mariners have been bad at player development for decades...which is absolutely true and is now absolutely irrelevant. This is a very different development system under Dipoto. He already had a ton of success maxing out marginal prospects in LA, and his efforts are having a large positive impact on the results in the minors for the Mariners.
Also, I would say that Dipoto did the right thing trading a lot of low-level guys for guys who are closer...for two reasons. 1) We are in win-now mode and 2) if we're so terrible at player development before he arrives, he should absolutely consider our guys "damaged goods" and try to get as many "pretty much developed" players into the system as he can, and start fresh with new blood through the drafts.