You gave 6 actually, and even with Sheahan and Glendening as locks for two spots, it is an enormous leap of faith to think any of our prospects can fill the two top 6 C spots. Sheahan is maybe close to be a 2/3 tweener, but doesn't look like he'll go any higher. AA and Nosek both had mediocre first years, slightly better than Helm and Emmerton, slightly worse than Abby. Hardly very promising. Holmstrom wasn't anything special, good playoff run aside, and has yet to play in NA.
Dekeyser is already closer to being a top pair D than any of our centers are to being top 6. Smith is at least 3rd pair. XO and Marchenko both spent time in the NHL, and looked decent. Both look more like good NHL players than AA, Nosek, or Holmstrom have shown. Sproul and Jensen have more questions, but I wouldn't say they're any worse than our centers.
All we really have at center is Larkin being way over-hyped. Just like we had people wanting to buy out Franzen to give Mantha a spot on the Wings before this year. He had a good year, bot not amazing or anything. It's a huge gamble to assume he's the answer to our future center needs.
It seems difficult to compare AA/Nosek vs. XO/Marchenko... XO and Marchenko (2nd year in GR) are a season ahead of AA and Nosek (1st year in GR). And, XO and Marchenko have had ample opportunity to show themselves at the NHL level, AA and Nosek have had no such opportunity.
Also, you said Mantha had a good year, but AA's year was just mediocre. I do not agree with this differentiation. Could you elaborate on why you feel that Mantha's season was better than AA's?