Prospects can be ruined by throwing them in the NHL before they are ready. Prospects don't get ruined by allowing them to develop along slowly.
There will be plenty of spots throughout the year with injuries and trades. Few spots are going to be open on day one, but there will be many open throughout the season.
Yes they do. If they're not learning anything new or get pigeon-holed into a system and role, they certainly aren't developing their game. Moreover, a player adjusts their body and game to the pace of their league. When getting sheltered in the NHL, a player is reminded everyday what further requirements are needed to play at a high level in the NHL. When they only get reminded once a year at training camp, the influence is only short term.
Opportunity plays a role in development. If you know there's no chance a team will allow you to prove yourself and learn from your mistakes, then this reduces motivation. Opting to play proven NHLers is what hurt the Flames in years past. Guys like St.Louis, Savard, Lombardi, etc. had better years when given a chance on other teams.
What's Monahan going to learn playing in a league he's already dominated when he needs to be challenged at a higher level so he can feel the pressure everyday to improve his game. I'm not talking about a top 6 role, I'm talking about sheltered offensive starts, opportunity to make mistakes, and chances on special teams. Guys like Stajan, Cammy, Stempniak and Jones aren't going to hold down the top 6 forever. They're here now so Monahan and other young guys aren't thrown to the wolves.