I think Vet vs Prospect depends on #1 is the prospect ready for NHL ice time and #2 their potential deployment.
If you say line 1 gets 20ish minutes a night and you have 40 to spread over the bottom 3. Are we talking lines 2/3 getting 15ish and the 4th getting 10 or less.
If a kid is getting less than 10 min a night at the NHL level but closer to 20 at the AHL level and is someone your project as having offensive upside then I think it benefits them to stay down.
If a kid will get closer to 15 a night on average and be put in at least some offensive situations then I think its more beneficial to have them stay up.
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Kasper, MBN, Danielson, and Mazur are all guys that I think project to play top 9 ideally as they develop. Could they probably all play 4th line right now, I'd say probably yes. I'd not a huge fan of sticking guys you want to develop offensively in shut down roles, as I do think it stymies them a bit offensively. I felt this help back Filppula a bit offensively when he came up.
So the question is can they beat out a guy for a 3rd line or above offensive spot. That also means whatever line Copp is on should be a 4th line because offensively the puck dies with him.
Larkin, Seider, Ray, DBC, Kane, Tank are locks for PP time. There are 4 spots available for PP time, so definitely an opportunity there. (Need to outplay Compher, Gus, Ed, Berggy).
Danielson, Kasper, and Mazur are also strong enough defensively to get a look at the PK which is also a big deal for a young guy. Often they can't be trusted to PK which allows a specialty guy like a Motte to be held onto vs playing the young guy.
Also while I don't want them on a 4th line permanently it is a positive that they can all play a shutdown role if needed because it allows them to slide down the lineup a bit without having to be fully sat if they go through some struggles. If they were offensive only guys, now you are looking at benching them or sending them down which is the kind of yoyoing that isn't good for guys.
One of these guys should make the team assuming they continue to look very solid like the older 3 have. MBN I fully expect play in the minors/SEL as he'd basically need to show that he is top 6 ready to make the team. Wings have plenty of offensive depth in GR for injuries/ callups, as 2 of the guys would be in GR. All 3 of the older guys have an NHL frame now, are showing they can handle NHL physicality, and are defensively responsible.