The only thing that will keep a good developer is interesting problems. If your problems are not interesting, they'll go away. The article also mentions they leave for better pay, so, I suspect they should be paying more.

Remember - being trained to work in their internal product is not really training they can use elsewhere. The company isn't doing them a favor by training them.

Also, if their programmers think building the system again from scratch is a good idea, perhaps they should consider it. "we have invested so-and-so millions in this system and we won't scrap it regardless of how obsolete it is" is sure to drive talent away.