I don't know that we know Nabby is not a good goalie coach? His entire body of NHL work is dealing with Martin Jones, and a dramatically improved (but still subpar/mediocre) not to mention tiny Aaron Dell.
You mention Korn and Greco, but how many goalie coaches are there at that level? Not many. Nabby spent nearly his career working with Tretiak and Strelow. I like to think some things probably rubbed off. Proximity of a player to elite coaching certainly doesn't guarantee that Nabby will be a good coach, but finding a guy like Korn is remarkably difficult.
Not to put you on the spot, but can you think of any other coaches who are worth that level?
On Jones, the guy was tremendous for a brief time, then got a huge contract then (almost certainly) had a key injury while having also having to deal with reduced pad sizes. He was a young league average (.912 or better) goalie who suddenly became very good in the playoffs, with 2 years of .923+ goaltending before singing his 6(+1) extension. Nabby and Niemi three playoff seasons at .920 between them.
Jones deserved his contract, the fact the world collapsed after signing it is just brutally unfortunate. You could say Wilson should have waited a year, but Jones followed the two strong years with another strong one, so if anything the contract could have been worse. I guess the learning is you simply can't invest in goaltending, even as a contending team? Or perhaps, don't invest in goaltending until you have elite centers, a very good defense, a good coach, and loads of depth?