You could argue that many of these guys still need to make an income. A lot of these guys did not make enough money to retire at 35-40. Many might be making more as a goalie coach than they did as an AHL goalie.
Great or not, goalie is a very technical position. Maybe a guy like Hasek couldn't teach it, but some of the best probably could just fine. Do they want to or need to is another question.
I also don't necessarily think great NHL players can't be good coaches. It seems like a giant myth. It's just that they like to take short cuts. Did Wayne Gretzky learn the craft before jumping into an NHL job. Not really. Randy Carlyle was a damn good NHL player, but he coaches for a decade (including 6 or so as a head coach in the AHL) before becoming an NHL head coach.
I just think it can be harder for a great, naturally gifted player to communicate his knowledge. Not always, because clearly Robinson is a fantastic defensive coach. Darryl Sutter was also a pretty good player and a great coach. But a guy like Stevens, who was known to lead by example and not through communication, might have a harder time verbalizing.
I'd also say that while some of those goalies I listed may have gotten their jobs out of necessity, I'm sure they kept those jobs by being good at what they do.